diff options
Diffstat (limited to 'Documentation/locking')
| -rw-r--r-- | Documentation/locking/lockdep-design.txt | 286 | ||||
| -rw-r--r-- | Documentation/locking/lockstat.txt | 178 | ||||
| -rw-r--r-- | Documentation/locking/mutex-design.txt | 157 | ||||
| -rw-r--r-- | Documentation/locking/rt-mutex-design.txt | 781 | ||||
| -rw-r--r-- | Documentation/locking/rt-mutex.txt | 79 | ||||
| -rw-r--r-- | Documentation/locking/spinlocks.txt | 167 | ||||
| -rw-r--r-- | Documentation/locking/ww-mutex-design.txt | 344 |
7 files changed, 1992 insertions, 0 deletions
diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt new file mode 100644 index 000000000000..5dbc99c04f6e --- /dev/null +++ b/Documentation/locking/lockdep-design.txt @@ -0,0 +1,286 @@ +Runtime locking correctness validator +===================================== + +started by Ingo Molnar <mingo@redhat.com> +additions by Arjan van de Ven <arjan@linux.intel.com> + +Lock-class +---------- + +The basic object the validator operates upon is a 'class' of locks. + +A class of locks is a group of locks that are logically the same with +respect to locking rules, even if the locks may have multiple (possibly +tens of thousands of) instantiations. For example a lock in the inode +struct is one class, while each inode has its own instantiation of that +lock class. + +The validator tracks the 'state' of lock-classes, and it tracks +dependencies between different lock-classes. The validator maintains a +rolling proof that the state and the dependencies are correct. + +Unlike an lock instantiation, the lock-class itself never goes away: when +a lock-class is used for the first time after bootup it gets registered, +and all subsequent uses of that lock-class will be attached to this +lock-class. + +State +----- + +The validator tracks lock-class usage history into 4n + 1 separate state bits: + +- 'ever held in STATE context' +- 'ever held as readlock in STATE context' +- 'ever held with STATE enabled' +- 'ever held as readlock with STATE enabled' + +Where STATE can be either one of (kernel/lockdep_states.h) + - hardirq + - softirq + - reclaim_fs + +- 'ever used' [ == !unused ] + +When locking rules are violated, these state bits are presented in the +locking error messages, inside curlies. A contrived example: + + modprobe/2287 is trying to acquire lock: + (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24 + + but task is already holding lock: + (&sio_locks[i].lock){-.-...}, at: [<c02867fd>] mutex_lock+0x21/0x24 + + +The bit position indicates STATE, STATE-read, for each of the states listed +above, and the character displayed in each indicates: + + '.' acquired while irqs disabled and not in irq context + '-' acquired in irq context + '+' acquired with irqs enabled + '?' acquired in irq context with irqs enabled. + +Unused mutexes cannot be part of the cause of an error. + + +Single-lock state rules: +------------------------ + +A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The +following states are exclusive, and only one of them is allowed to be +set for any lock-class: + + <hardirq-safe> and <hardirq-unsafe> + <softirq-safe> and <softirq-unsafe> + +The validator detects and reports lock usage that violate these +single-lock state rules. + +Multi-lock dependency rules: +---------------------------- + +The same lock-class must not be acquired twice, because this could lead +to lock recursion deadlocks. + +Furthermore, two locks may not be taken in different order: + + <L1> -> <L2> + <L2> -> <L1> + +because this could lead to lock inversion deadlocks. (The validator +finds such dependencies in arbitrary complexity, i.e. there can be any +other locking sequence between the acquire-lock operations, the +validator will still track all dependencies between locks.) + +Furthermore, the following usage based lock dependencies are not allowed +between any two lock-classes: + + <hardirq-safe> -> <hardirq-unsafe> + <softirq-safe> -> <softirq-unsafe> + +The first rule comes from the fact the a hardirq-safe lock could be +taken by a hardirq context, interrupting a hardirq-unsafe lock - and +thus could result in a lock inversion deadlock. Likewise, a softirq-safe +lock could be taken by an softirq context, interrupting a softirq-unsafe +lock. + +The above rules are enforced for any locking sequence that occurs in the +kernel: when acquiring a new lock, the validator checks whether there is +any rule violation between the new lock and any of the held locks. + +When a lock-class changes its state, the following aspects of the above +dependency rules are enforced: + +- if a new hardirq-safe lock is discovered, we check whether it + took any hardirq-unsafe lock in the past. + +- if a new softirq-safe lock is discovered, we check whether it took + any softirq-unsafe lock in the past. + +- if a new hardirq-unsafe lock is discovered, we check whether any + hardirq-safe lock took it in the past. + +- if a new softirq-unsafe lock is discovered, we check whether any + softirq-safe lock took it in the past. + +(Again, we do these checks too on the basis that an interrupt context +could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which +could lead to a lock inversion deadlock - even if that lock scenario did +not trigger in practice yet.) + +Exception: Nested data dependencies leading to nested locking +------------------------------------------------------------- + +There are a few cases where the Linux kernel acquires more than one +instance of the same lock-class. Such cases typically happen when there +is some sort of hierarchy within objects of the same type. In these +cases there is an inherent "natural" ordering between the two objects +(defined by the properties of the hierarchy), and the kernel grabs the +locks in this fixed order on each of the objects. + +An example of such an object hierarchy that results in "nested locking" +is that of a "whole disk" block-dev object and a "partition" block-dev +object; the partition is "part of" the whole device and as long as one +always takes the whole disk lock as a higher lock than the partition +lock, the lock ordering is fully correct. The validator does not +automatically detect this natural ordering, as the locking rule behind +the ordering is not static. + +In order to teach the validator about this correct usage model, new +versions of the various locking primitives were added that allow you to +specify a "nesting level". An example call, for the block device mutex, +looks like this: + +enum bdev_bd_mutex_lock_class +{ + BD_MUTEX_NORMAL, + BD_MUTEX_WHOLE, + BD_MUTEX_PARTITION +}; + + mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION); + +In this case the locking is done on a bdev object that is known to be a +partition. + +The validator treats a lock that is taken in such a nested fashion as a +separate (sub)class for the purposes of validation. + +Note: When changing code to use the _nested() primitives, be careful and +check really thoroughly that the hierarchy is correctly mapped; otherwise +you can get false positives or false negatives. + +Proof of 100% correctness: +-------------------------- + +The validator achieves perfect, mathematical 'closure' (proof of locking +correctness) in the sense that for every simple, standalone single-task +locking sequence that occurred at least once during the lifetime of the +kernel, the validator proves it with a 100% certainty that no +combination and timing of these locking sequences can cause any class of +lock related deadlock. [*] + +I.e. complex multi-CPU and multi-task locking scenarios do not have to +occur in practice to prove a deadlock: only the simple 'component' +locking chains have to occur at least once (anytime, in any +task/context) for the validator to be able to prove correctness. (For +example, complex deadlocks that would normally need more than 3 CPUs and +a very unlikely constellation of tasks, irq-contexts and timings to +occur, can be detected on a plain, lightly loaded single-CPU system as +well!) + +This radically decreases the complexity of locking related QA of the +kernel: what has to be done during QA is to trigger as many "simple" +single-task locking dependencies in the kernel as possible, at least +once, to prove locking correctness - instead of having to trigger every +possible combination of locking interaction between CPUs, combined with +every possible hardirq and softirq nesting scenario (which is impossible +to do in practice). + +[*] assuming that the validator itself is 100% correct, and no other + part of the system corrupts the state of the validator in any way. + We also assume that all NMI/SMM paths [which could interrupt + even hardirq-disabled codepaths] are correct and do not interfere + with the validator. We also assume that the 64-bit 'chain hash' + value is unique for every lock-chain in the system. Also, lock + recursion must not be higher than 20. + +Performance: +------------ + +The above rules require _massive_ amounts of runtime checking. If we did +that for every lock taken and for every irqs-enable event, it would +render the system practically unusably slow. The complexity of checking +is O(N^2), so even with just a few hundred lock-classes we'd have to do +tens of thousands of checks for every event. + +This problem is solved by checking any given 'locking scenario' (unique +sequence of locks taken after each other) only once. A simple stack of +held locks is maintained, and a lightweight 64-bit hash value is +calculated, which hash is unique for every lock chain. The hash value, +when the chain is validated for the first time, is then put into a hash +table, which hash-table can be checked in a lockfree manner. If the +locking chain occurs again later on, the hash table tells us that we +dont have to validate the chain again. + +Troubleshooting: +---------------- + +The validator tracks a maximum of MAX_LOCKDEP_KEYS number of lock classes. +Exceeding this number will trigger the following lockdep warning: + + (DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS)) + +By default, MAX_LOCKDEP_KEYS is currently set to 8191, and typical +desktop systems have less than 1,000 lock classes, so this warning +normally results from lock-class leakage or failure to properly +initialize locks. These two problems are illustrated below: + +1. Repeated module loading and unloading while running the validator + will result in lock-class leakage. The issue here is that each + load of the module will create a new set of lock classes for + that module's locks, but module unloading does not remove old + classes (see below discussion of reuse of lock classes for why). + Therefore, if that module is loaded and unloaded repeatedly, + the number of lock classes will eventually reach the maximum. + +2. Using structures such as arrays that have large numbers of + locks that are not explicitly initialized. For example, + a hash table with 8192 buckets where each bucket has its own + spinlock_t will consume 8192 lock classes -unless- each spinlock + is explicitly initialized at runtime, for example, using the + run-time spin_lock_init() as opposed to compile-time initializers + such as __SPIN_LOCK_UNLOCKED(). Failure to properly initialize + the per-bucket spinlocks would guarantee lock-class overflow. + In contrast, a loop that called spin_lock_init() on each lock + would place all 8192 locks into a single lock class. + + The moral of this story is that you should always explicitly + initialize your locks. + +One might argue that the validator should be modified to allow +lock classes to be reused. However, if you are tempted to make this +argument, first review the code and think through the changes that would +be required, keeping in mind that the lock classes to be removed are +likely to be linked into the lock-dependency graph. This turns out to +be harder to do than to say. + +Of course, if you do run out of lock classes, the next thing to do is +to find the offending lock classes. First, the following command gives +you the number of lock classes currently in use along with the maximum: + + grep "lock-classes" /proc/lockdep_stats + +This command produces the following output on a modest system: + + lock-classes: 748 [max: 8191] + +If the number allocated (748 above) increases continually over time, +then there is likely a leak. The following command can be used to +identify the leaking lock classes: + + grep "BD" /proc/lockdep + +Run the command and save the output, then compare against the output from +a later run of this command to identify the leakers. This same output +can also help you find situations where runtime lock initialization has +been omitted. diff --git a/Documentation/locking/lockstat.txt b/Documentation/locking/lockstat.txt new file mode 100644 index 000000000000..7428773a1e69 --- /dev/null +++ b/Documentation/locking/lockstat.txt @@ -0,0 +1,178 @@ + +LOCK STATISTICS + +- WHAT + +As the name suggests, it provides statistics on locks. + +- WHY + +Because things like lock contention can severely impact performance. + +- HOW + +Lockdep already has hooks in the lock functions and maps lock instances to +lock classes. We build on that (see Documentation/lokcing/lockdep-design.txt). +The graph below shows the relation between the lock functions and the various +hooks therein. + + __acquire + | + lock _____ + | \ + | __contended + | | + | <wait> + | _______/ + |/ + | + __acquired + | + . + <hold> + . + | + __release + | + unlock + +lock, unlock - the regular lock functions +__* - the hooks +<> - states + +With these hooks we provide the following statistics: + + con-bounces - number of lock contention that involved x-cpu data + contentions - number of lock acquisitions that had to wait + wait time min - shortest (non-0) time we ever had to wait for a lock + max - longest time we ever had to wait for a lock + total - total time we spend waiting on this lock + avg - average time spent waiting on this lock + acq-bounces - number of lock acquisitions that involved x-cpu data + acquisitions - number of times we took the lock + hold time min - shortest (non-0) time we ever held the lock + max - longest time we ever held the lock + total - total time this lock was held + avg - average time this lock was held + +These numbers are gathered per lock class, per read/write state (when +applicable). + +It also tracks 4 contention points per class. A contention point is a call site +that had to wait on lock acquisition. + + - CONFIGURATION + +Lock statistics are enabled via CONFIG_LOCK_STAT. + + - USAGE + +Enable collection of statistics: + +# echo 1 >/proc/sys/kernel/lock_stat + +Disable collection of statistics: + +# echo 0 >/proc/sys/kernel/lock_stat + +Look at the current lock statistics: + +( line numbers not part of actual output, done for clarity in the explanation + below ) + +# less /proc/lock_stat + +01 lock_stat version 0.4 +02----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- +03 class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg +04----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- +05 +06 &mm->mmap_sem-W: 46 84 0.26 939.10 16371.53 194.90 47291 2922365 0.16 2220301.69 17464026916.32 5975.99 +07 &mm->mmap_sem-R: 37 100 1.31 299502.61 325629.52 3256.30 212344 34316685 0.10 7744.91 95016910.20 2.77 +08 --------------- +09 &mm->mmap_sem 1 [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280 +19 &mm->mmap_sem 96 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510 +11 &mm->mmap_sem 34 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0 +12 &mm->mmap_sem 17 [<ffffffff81127e71>] vm_munmap+0x41/0x80 +13 --------------- +14 &mm->mmap_sem 1 [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0 +15 &mm->mmap_sem 60 [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250 +16 &mm->mmap_sem 41 [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510 +17 &mm->mmap_sem 68 [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0 +18 +19............................................................................................................................................................................................................................. +20 +21 unix_table_lock: 110 112 0.21 49.24 163.91 1.46 21094 66312 0.12 624.42 31589.81 0.48 +22 --------------- +23 unix_table_lock 45 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0 +24 unix_table_lock 47 [<ffffffff8150b111>] unix_release_sock+0x31/0x250 +25 unix_table_lock 15 [<ffffffff8150ca37>] unix_find_other+0x117/0x230 +26 unix_table_lock 5 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0 +27 --------------- +28 unix_table_lock 39 [<ffffffff8150b111>] unix_release_sock+0x31/0x250 +29 unix_table_lock 49 [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0 +30 unix_table_lock 20 [<ffffffff8150ca37>] unix_find_other+0x117/0x230 +31 unix_table_lock 4 [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0 + + +This excerpt shows the first two lock class statistics. Line 01 shows the +output version - each time the format changes this will be updated. Line 02-04 +show the header with column descriptions. Lines 05-18 and 20-31 show the actual +statistics. These statistics come in two parts; the actual stats separated by a +short separator (line 08, 13) from the contention points. + +The first lock (05-18) is a read/write lock, and shows two lines above the +short separator. The contention points don't match the column descriptors, +they have two: contentions and [<IP>] symbol. The second set of contention +points are the points we're contending with. + +The integer part of the time values is in us. + +Dealing with nested locks, subclasses may appear: + +32........................................................................................................................................................................................................................... +33 +34 &rq->lock: 13128 13128 0.43 190.53 103881.26 7.91 97454 3453404 0.00 401.11 13224683.11 3.82 +35 --------- +36 &rq->lock 645 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 +37 &rq->lock 297 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a +38 &rq->lock 360 [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a +39 &rq->lock 428 [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb +40 --------- +41 &rq->lock 77 [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75 +42 &rq->lock 174 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a +43 &rq->lock 4715 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 +44 &rq->lock 893 [<ffffffff81340524>] schedule+0x157/0x7b8 +45 +46........................................................................................................................................................................................................................... +47 +48 &rq->lock/1: 1526 11488 0.33 388.73 136294.31 11.86 21461 38404 0.00 37.93 109388.53 2.84 +49 ----------- +50 &rq->lock/1 11526 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 +51 ----------- +52 &rq->lock/1 5645 [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54 +53 &rq->lock/1 1224 [<ffffffff81340524>] schedule+0x157/0x7b8 +54 &rq->lock/1 4336 [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54 +55 &rq->lock/1 181 [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a + +Line 48 shows statistics for the second subclass (/1) of &rq->lock class +(subclass starts from 0), since in this case, as line 50 suggests, +double_rq_lock actually acquires a nested lock of two spinlocks. + +View the top contending locks: + +# grep : /proc/lock_stat | head + clockevents_lock: 2926159 2947636 0.15 46882.81 1784540466.34 605.41 3381345 3879161 0.00 2260.97 53178395.68 13.71 + tick_broadcast_lock: 346460 346717 0.18 2257.43 39364622.71 113.54 3642919 4242696 0.00 2263.79 49173646.60 11.59 + &mapping->i_mmap_mutex: 203896 203899 3.36 645530.05 31767507988.39 155800.21 3361776 8893984 0.17 2254.15 14110121.02 1.59 + &rq->lock: 135014 136909 0.18 606.09 842160.68 6.15 1540728 10436146 0.00 728.72 17606683.41 1.69 + &(&zone->lru_lock)->rlock: 93000 94934 0.16 59.18 188253.78 1.98 1199912 3809894 0.15 391.40 3559518.81 0.93 + tasklist_lock-W: 40667 41130 0.23 1189.42 428980.51 10.43 270278 510106 0.16 653.51 3939674.91 7.72 + tasklist_lock-R: 21298 21305 0.20 1310.05 215511.12 10.12 186204 241258 0.14 1162.33 1179779.23 4.89 + rcu_node_1: 47656 49022 0.16 635.41 193616.41 3.95 844888 1865423 0.00 764.26 1656226.96 0.89 + &(&dentry->d_lockref.lock)->rlock: 39791 40179 0.15 1302.08 88851.96 2.21 2790851 12527025 0.10 1910.75 3379714.27 0.27 + rcu_node_0: 29203 30064 0.16 786.55 1555573.00 51.74 88963 244254 0.00 398.87 428872.51 1.76 + +Clear the statistics: + +# echo 0 > /proc/lock_stat diff --git a/Documentation/locking/mutex-design.txt b/Documentation/locking/mutex-design.txt new file mode 100644 index 000000000000..ee231ed09ec6 --- /dev/null +++ b/Documentation/locking/mutex-design.txt @@ -0,0 +1,157 @@ +Generic Mutex Subsystem + +started by Ingo Molnar <mingo@redhat.com> +updated by Davidlohr Bueso <davidlohr@hp.com> + +What are mutexes? +----------------- + +In the Linux kernel, mutexes refer to a particular locking primitive +that enforces serialization on shared memory systems, and not only to +the generic term referring to 'mutual exclusion' found in academia +or similar theoretical text books. Mutexes are sleeping locks which +behave similarly to binary semaphores, and were introduced in 2006[1] +as an alternative to these. This new data structure provided a number +of advantages, including simpler interfaces, and at that time smaller +code (see Disadvantages). + +[1] http://lwn.net/Articles/164802/ + +Implementation +-------------- + +Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h +and implemented in kernel/locking/mutex.c. These locks use a three +state atomic counter (->count) to represent the different possible +transitions that can occur during the lifetime of a lock: + + 1: unlocked + 0: locked, no waiters + negative: locked, with potential waiters + +In its most basic form it also includes a wait-queue and a spinlock +that serializes access to it. CONFIG_SMP systems can also include +a pointer to the lock task owner (->owner) as well as a spinner MCS +lock (->osq), both described below in (ii). + +When acquiring a mutex, there are three possible paths that can be +taken, depending on the state of the lock: + +(i) fastpath: tries to atomically acquire the lock by decrementing the + counter. If it was already taken by another task it goes to the next + possible path. This logic is architecture specific. On x86-64, the + locking fastpath is 2 instructions: + + 0000000000000e10 <mutex_lock>: + e21: f0 ff 0b lock decl (%rbx) + e24: 79 08 jns e2e <mutex_lock+0x1e> + + the unlocking fastpath is equally tight: + + 0000000000000bc0 <mutex_unlock>: + bc8: f0 ff 07 lock incl (%rdi) + bcb: 7f 0a jg bd7 <mutex_unlock+0x17> + + +(ii) midpath: aka optimistic spinning, tries to spin for acquisition + while the lock owner is running and there are no other tasks ready + to run that have higher priority (need_resched). The rationale is + that if the lock owner is running, it is likely to release the lock + soon. The mutex spinners are queued up using MCS lock so that only + one spinner can compete for the mutex. + + The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock + with the desirable properties of being fair and with each cpu trying + to acquire the lock spinning on a local variable. It avoids expensive + cacheline bouncing that common test-and-set spinlock implementations + incur. An MCS-like lock is specially tailored for optimistic spinning + for sleeping lock implementation. An important feature of the customized + MCS lock is that it has the extra property that spinners are able to exit + the MCS spinlock queue when they need to reschedule. This further helps + avoid situations where MCS spinners that need to reschedule would continue + waiting to spin on mutex owner, only to go directly to slowpath upon + obtaining the MCS lock. + + +(iii) slowpath: last resort, if the lock is still unable to be acquired, + the task is added to the wait-queue and sleeps until woken up by the + unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE. + +While formally kernel mutexes are sleepable locks, it is path (ii) that +makes them more practically a hybrid type. By simply not interrupting a +task and busy-waiting for a few cycles instead of immediately sleeping, +the performance of this lock has been seen to significantly improve a +number of workloads. Note that this technique is also used for rw-semaphores. + +Semantics +--------- + +The mutex subsystem checks and enforces the following rules: + + - Only one task can hold the mutex at a time. + - Only the owner can unlock the mutex. + - Multiple unlocks are not permitted. + - Recursive locking/unlocking is not permitted. + - A mutex must only be initialized via the API (see below). + - A task may not exit with a mutex held. + - Memory areas where held locks reside must not be freed. + - Held mutexes must not be reinitialized. + - Mutexes may not be used in hardware or software interrupt + contexts such as tasklets and timers. + +These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled. +In addition, the mutex debugging code also implements a number of other +features that make lock debugging easier and faster: + + - Uses symbolic names of mutexes, whenever they are printed + in debug output. + - Point-of-acquire tracking, symbolic lookup of function names, + list of all locks held in the system, printout of them. + - Owner tracking. + - Detects self-recursing locks and prints out all relevant info. + - Detects multi-task circular deadlocks and prints out all affected + locks and tasks (and only those tasks). + + +Interfaces +---------- +Statically define the mutex: + DEFINE_MUTEX(name); + +Dynamically initialize the mutex: + mutex_init(mutex); + +Acquire the mutex, uninterruptible: + void mutex_lock(struct mutex *lock); + void mutex_lock_nested(struct mutex *lock, unsigned int subclass); + int mutex_trylock(struct mutex *lock); + +Acquire the mutex, interruptible: + int mutex_lock_interruptible_nested(struct mutex *lock, + unsigned int subclass); + int mutex_lock_interruptible(struct mutex *lock); + +Acquire the mutex, interruptible, if dec to 0: + int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock); + +Unlock the mutex: + void mutex_unlock(struct mutex *lock); + +Test if the mutex is taken: + int mutex_is_locked(struct mutex *lock); + +Disadvantages +------------- + +Unlike its original design and purpose, 'struct mutex' is larger than +most locks in the kernel. E.g: on x86-64 it is 40 bytes, almost twice +as large as 'struct semaphore' (24 bytes) and 8 bytes shy of the +'struct rw_semaphore' variant. Larger structure sizes mean more CPU +cache and memory footprint. + +When to use mutexes +------------------- + +Unless the strict semantics of mutexes are unsuitable and/or the critical +region prevents the lock from being shared, always prefer them to any other +locking primitive. diff --git a/Documentation/locking/rt-mutex-design.txt b/Documentation/locking/rt-mutex-design.txt new file mode 100644 index 000000000000..8666070d3189 --- /dev/null +++ b/Documentation/locking/rt-mutex-design.txt @@ -0,0 +1,781 @@ +# +# Copyright (c) 2006 Steven Rostedt +# Licensed under the GNU Free Documentation License, Version 1.2 +# + +RT-mutex implementation design +------------------------------ + +This document tries to describe the design of the rtmutex.c implementation. +It doesn't describe the reasons why rtmutex.c exists. For that please see +Documentation/rt-mutex.txt. Although this document does explain problems +that happen without this code, but that is in the concept to understand +what the code actually is doing. + +The goal of this document is to help others understand the priority +inheritance (PI) algorithm that is used, as well as reasons for the +decisions that were made to implement PI in the manner that was done. + + +Unbounded Priority Inversion +---------------------------- + +Priority inversion is when a lower priority process executes while a higher +priority process wants to run. This happens for several reasons, and +most of the time it can't be helped. Anytime a high priority process wants +to use a resource that a lower priority process has (a mutex for example), +the high priority process must wait until the lower priority process is done +with the resource. This is a priority inversion. What we want to prevent +is something called unbounded priority inversion. That is when the high +priority process is prevented from running by a lower priority process for +an undetermined amount of time. + +The classic example of unbounded priority inversion is where you have three +processes, let's call them processes A, B, and C, where A is the highest +priority process, C is the lowest, and B is in between. A tries to grab a lock +that C owns and must wait and lets C run to release the lock. But in the +meantime, B executes, and since B is of a higher priority than C, it preempts C, +but by doing so, it is in fact preempting A which is a higher priority process. +Now there's no way of knowing how long A will be sleeping waiting for C +to release the lock, because for all we know, B is a CPU hog and will +never give C a chance to release the lock. This is called unbounded priority +inversion. + +Here's a little ASCII art to show the problem. + + grab lock L1 (owned by C) + | +A ---+ + C preempted by B + | +C +----+ + +B +--------> + B now keeps A from running. + + +Priority Inheritance (PI) +------------------------- + +There are several ways to solve this issue, but other ways are out of scope +for this document. Here we only discuss PI. + +PI is where a process inherits the priority of another process if the other +process blocks on a lock owned by the current process. To make this easier +to understand, let's use the previous example, with processes A, B, and C again. + +This time, when A blocks on the lock owned by C, C would inherit the priority +of A. So now if B becomes runnable, it would not preempt C, since C now has +the high priority of A. As soon as C releases the lock, it loses its +inherited priority, and A then can continue with the resource that C had. + +Terminology +----------- + +Here I explain some terminology that is used in this document to help describe +the design that is used to implement PI. + +PI chain - The PI chain is an ordered series of locks and processes that cause + processes to inherit priorities from a previous process that is + blocked on one of its locks. This is described in more detail + later in this document. + +mutex - In this document, to differentiate from locks that implement + PI and spin locks that are used in the PI code, from now on + the PI locks will be called a mutex. + +lock - In this document from now on, I will use the term lock when + referring to spin locks that are used to protect parts of the PI + algorithm. These locks disable preemption for UP (when + CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from + entering critical sections simultaneously. + +spin lock - Same as lock above. + +waiter - A waiter is a struct that is stored on the stack of a blocked + process. Since the scope of the waiter is within the code for + a process being blocked on the mutex, it is fine to allocate + the waiter on the process's stack (local variable). This + structure holds a pointer to the task, as well as the mutex that + the task is blocked on. It also has the plist node structures to + place the task in the waiter_list of a mutex as well as the + pi_list of a mutex owner task (described below). + + waiter is sometimes used in reference to the task that is waiting + on a mutex. This is the same as waiter->task. + +waiters - A list of processes that are blocked on a mutex. + +top waiter - The highest priority process waiting on a specific mutex. + +top pi waiter - The highest priority process waiting on one of the mutexes + that a specific process owns. + +Note: task and process are used interchangeably in this document, mostly to + differentiate between two processes that are being described together. + + +PI chain +-------- + +The PI chain is a list of processes and mutexes that may cause priority +inheritance to take place. Multiple chains may converge, but a chain +would never diverge, since a process can't be blocked on more than one +mutex at a time. + +Example: + + Process: A, B, C, D, E + Mutexes: L1, L2, L3, L4 + + A owns: L1 + B blocked on L1 + B owns L2 + C blocked on L2 + C owns L3 + D blocked on L3 + D owns L4 + E blocked on L4 + +The chain would be: + + E->L4->D->L3->C->L2->B->L1->A + +To show where two chains merge, we could add another process F and +another mutex L5 where B owns L5 and F is blocked on mutex L5. + +The chain for F would be: + + F->L5->B->L1->A + +Since a process may own more than one mutex, but never be blocked on more than +one, the chains merge. + +Here we show both chains: + + E->L4->D->L3->C->L2-+ + | + +->B->L1->A + | + F->L5-+ + +For PI to work, the processes at the right end of these chains (or we may +also call it the Top of the chain) must be equal to or higher in priority +than the processes to the left or below in the chain. + +Also since a mutex may have more than one process blocked on it, we can +have multiple chains merge at mutexes. If we add another process G that is +blocked on mutex L2: + + G->L2->B->L1->A + +And once again, to show how this can grow I will show the merging chains +again. + + E->L4->D->L3->C-+ + +->L2-+ + | | + G-+ +->B->L1->A + | + F->L5-+ + + +Plist +----- + +Before I go further and talk about how the PI chain is stored through lists +on both mutexes and processes, I'll explain the plist. This is similar to +the struct list_head functionality that is already in the kernel. +The implementation of plist is out of scope for this document, but it is +very important to understand what it does. + +There are a few differences between plist and list, the most important one +being that plist is a priority sorted linked list. This means that the +priorities of the plist are sorted, such that it takes O(1) to retrieve the +highest priority item in the list. Obviously this is useful to store processes +based on their priorities. + +Another difference, which is important for implementation, is that, unlike +list, the head of the list is a different element than the nodes of a list. +So the head of the list is declared as struct plist_head and nodes that will +be added to the list are declared as struct plist_node. + + +Mutex Waiter List +----------------- + +Every mutex keeps track of all the waiters that are blocked on itself. The mutex +has a plist to store these waiters by priority. This list is protected by +a spin lock that is located in the struct of the mutex. This lock is called +wait_lock. Since the modification of the waiter list is never done in +interrupt context, the wait_lock can be taken without disabling interrupts. + + +Task PI List +------------ + +To keep track of the PI chains, each process has its own PI list. This is +a list of all top waiters of the mutexes that are owned by the process. +Note that this list only holds the top waiters and not all waiters that are +blocked on mutexes owned by the process. + +The top of the task's PI list is always the highest priority task that +is waiting on a mutex that is owned by the task. So if the task has +inherited a priority, it will always be the priority of the task that is +at the top of this list. + +This list is stored in the task structure of a process as a plist called +pi_list. This list is protected by a spin lock also in the task structure, +called pi_lock. This lock may also be taken in interrupt context, so when +locking the pi_lock, interrupts must be disabled. + + +Depth of the PI Chain +--------------------- + +The maximum depth of the PI chain is not dynamic, and could actually be +defined. But is very complex to figure it out, since it depends on all +the nesting of mutexes. Let's look at the example where we have 3 mutexes, +L1, L2, and L3, and four separate functions func1, func2, func3 and func4. +The following shows a locking order of L1->L2->L3, but may not actually +be directly nested that way. + +void func1(void) +{ + mutex_lock(L1); + + /* do anything */ + + mutex_unlock(L1); +} + +void func2(void) +{ + mutex_lock(L1); + mutex_lock(L2); + + /* do something */ + + mutex_unlock(L2); + mutex_unlock(L1); +} + +void func3(void) +{ + mutex_lock(L2); + mutex_lock(L3); + + /* do something else */ + + mutex_unlock(L3); + mutex_unlock(L2); +} + +void func4(void) +{ + mutex_lock(L3); + + /* do something again */ + + mutex_unlock(L3); +} + +Now we add 4 processes that run each of these functions separately. +Processes A, B, C, and D which run functions func1, func2, func3 and func4 +respectively, and such that D runs first and A last. With D being preempted +in func4 in the "do something again" area, we have a locking that follows: + +D owns L3 + C blocked on L3 + C owns L2 + B blocked on L2 + B owns L1 + A blocked on L1 + +And thus we have the chain A->L1->B->L2->C->L3->D. + +This gives us a PI depth of 4 (four processes), but looking at any of the +functions individually, it seems as though they only have at most a locking +depth of two. So, although the locking depth is defined at compile time, +it still is very difficult to find the possibilities of that depth. + +Now since mutexes can be defined by user-land applications, we don't want a DOS +type of application that nests large amounts of mutexes to create a large +PI chain, and have the code holding spin locks while looking at a large +amount of data. So to prevent this, the implementation not only implements +a maximum lock depth, but also only holds at most two different locks at a +time, as it walks the PI chain. More about this below. + + +Mutex owner and flags +--------------------- + +The mutex structure contains a pointer to the owner of the mutex. If the +mutex is not owned, this owner is set to NULL. Since all architectures +have the task structure on at least a four byte alignment (and if this is +not true, the rtmutex.c code will be broken!), this allows for the two +least significant bits to be used as flags. This part is also described +in Documentation/rt-mutex.txt, but will also be briefly described here. + +Bit 0 is used as the "Pending Owner" flag. This is described later. +Bit 1 is used as the "Has Waiters" flags. This is also described later + in more detail, but is set whenever there are waiters on a mutex. + + +cmpxchg Tricks +-------------- + +Some architectures implement an atomic cmpxchg (Compare and Exchange). This +is used (when applicable) to keep the fast path of grabbing and releasing +mutexes short. + +cmpxchg is basically the following function performed atomically: + +unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C) +{ + unsigned long T = *A; + if (*A == *B) { + *A = *C; + } + return T; +} +#define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c) + +This is really nice to have, since it allows you to only update a variable +if the variable is what you expect it to be. You know if it succeeded if +the return value (the old value of A) is equal to B. + +The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If +the architecture does not support CMPXCHG, then this macro is simply set +to fail every time. But if CMPXCHG is supported, then this will +help out extremely to keep the fast path short. + +The use of rt_mutex_cmpxchg with the flags in the owner field help optimize +the system for architectures that support it. This will also be explained +later in this document. + + +Priority adjustments +-------------------- + +The implementation of the PI code in rtmutex.c has several places that a +process must adjust its priority. With the help of the pi_list of a +process this is rather easy to know what needs to be adjusted. + +The functions implementing the task adjustments are rt_mutex_adjust_prio, +__rt_mutex_adjust_prio (same as the former, but expects the task pi_lock +to already be taken), rt_mutex_getprio, and rt_mutex_setprio. + +rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio. + +rt_mutex_getprio returns the priority that the task should have. Either the +task's own normal priority, or if a process of a higher priority is waiting on +a mutex owned by the task, then that higher priority should be returned. +Since the pi_list of a task holds an order by priority list of all the top +waiters of all the mutexes that the task owns, rt_mutex_getprio simply needs +to compare the top pi waiter to its own normal priority, and return the higher +priority back. + +(Note: if looking at the code, you will notice that the lower number of + prio is returned. This is because the prio field in the task structure + is an inverse order of the actual priority. So a "prio" of 5 is + of higher priority than a "prio" of 10.) + +__rt_mutex_adjust_prio examines the result of rt_mutex_getprio, and if the +result does not equal the task's current priority, then rt_mutex_setprio +is called to adjust the priority of the task to the new priority. +Note that rt_mutex_setprio is defined in kernel/sched/core.c to implement the +actual change in priority. + +It is interesting to note that __rt_mutex_adjust_prio can either increase +or decrease the priority of the task. In the case that a higher priority +process has just blocked on a mutex owned by the task, __rt_mutex_adjust_prio +would increase/boost the task's priority. But if a higher priority task +were for some reason to leave the mutex (timeout or signal), this same function +would decrease/unboost the priority of the task. That is because the pi_list +always contains the highest priority task that is waiting on a mutex owned +by the task, so we only need to compare the priority of that top pi waiter +to the normal priority of the given task. + + +High level overview of the PI chain walk +---------------------------------------- + +The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain. + +The implementation has gone through several iterations, and has ended up +with what we believe is the best. It walks the PI chain by only grabbing +at most two locks at a time, and is very efficient. + +The rt_mutex_adjust_prio_chain can be used either to boost or lower process +priorities. + +rt_mutex_adjust_prio_chain is called with a task to be checked for PI +(de)boosting (the owner of a mutex that a process is blocking on), a flag to +check for deadlocking, the mutex that the task owns, and a pointer to a waiter +that is the process's waiter struct that is blocked on the mutex (although this +parameter may be NULL for deboosting). + +For this explanation, I will not mention deadlock detection. This explanation +will try to stay at a high level. + +When this function is called, there are no locks held. That also means +that the state of the owner and lock can change when entered into this function. + +Before this function is called, the task has already had rt_mutex_adjust_prio +performed on it. This means that the task is set to the priority that it +should be at, but the plist nodes of the task's waiter have not been updated +with the new priorities, and that this task may not be in the proper locations +in the pi_lists and wait_lists that the task is blocked on. This function +solves all that. + +A loop is entered, where task is the owner to be checked for PI changes that +was passed by parameter (for the first iteration). The pi_lock of this task is +taken to prevent any more changes to the pi_list of the task. This also +prevents new tasks from completing the blocking on a mutex that is owned by this +task. + +If the task is not blocked on a mutex then the loop is exited. We are at +the top of the PI chain. + +A check is now done to see if the original waiter (the process that is blocked +on the current mutex) is the top pi waiter of the task. That is, is this +waiter on the top of the task's pi_list. If it is not, it either means that +there is another process higher in priority that is blocked on one of the +mutexes that the task owns, or that the waiter has just woken up via a signal +or timeout and has left the PI chain. In either case, the loop is exited, since +we don't need to do any more changes to the priority of the current task, or any +task that owns a mutex that this current task is waiting on. A priority chain +walk is only needed when a new top pi waiter is made to a task. + +The next check sees if the task's waiter plist node has the priority equal to +the priority the task is set at. If they are equal, then we are done with +the loop. Remember that the function started with the priority of the +task adjusted, but the plist nodes that hold the task in other processes +pi_lists have not been adjusted. + +Next, we look at the mutex that the task is blocked on. The mutex's wait_lock +is taken. This is done by a spin_trylock, because the locking order of the +pi_lock and wait_lock goes in the opposite direction. If we fail to grab the +lock, the pi_lock is released, and we restart the loop. + +Now that we have both the pi_lock of the task as well as the wait_lock of +the mutex the task is blocked on, we update the task's waiter's plist node +that is located on the mutex's wait_list. + +Now we release the pi_lock of the task. + +Next the owner of the mutex has its pi_lock taken, so we can update the +task's entry in the owner's pi_list. If the task is the highest priority +process on the mutex's wait_list, then we remove the previous top waiter +from the owner's pi_list, and replace it with the task. + +Note: It is possible that the task was the current top waiter on the mutex, + in which case the task is not yet on the pi_list of the waiter. This + is OK, since plist_del does nothing if the plist node is not on any + list. + +If the task was not the top waiter of the mutex, but it was before we +did the priority updates, that means we are deboosting/lowering the +task. In this case, the task is removed from the pi_list of the owner, +and the new top waiter is added. + +Lastly, we unlock both the pi_lock of the task, as well as the mutex's +wait_lock, and continue the loop again. On the next iteration of the +loop, the previous owner of the mutex will be the task that will be +processed. + +Note: One might think that the owner of this mutex might have changed + since we just grab the mutex's wait_lock. And one could be right. + The important thing to remember is that the owner could not have + become the task that is being processed in the PI chain, since + we have taken that task's pi_lock at the beginning of the loop. + So as long as there is an owner of this mutex that is not the same + process as the tasked being worked on, we are OK. + + Looking closely at the code, one might be confused. The check for the + end of the PI chain is when the task isn't blocked on anything or the + task's waiter structure "task" element is NULL. This check is + protected only by the task's pi_lock. But the code to unlock the mutex + sets the task's waiter structure "task" element to NULL with only + the protection of the mutex's wait_lock, which was not taken yet. + Isn't this a race condition if the task becomes the new owner? + + The answer is No! The trick is the spin_trylock of the mutex's + wait_lock. If we fail that lock, we release the pi_lock of the + task and continue the loop, doing the end of PI chain check again. + + In the code to release the lock, the wait_lock of the mutex is held + the entire time, and it is not let go when we grab the pi_lock of the + new owner of the mutex. So if the switch of a new owner were to happen + after the check for end of the PI chain and the grabbing of the + wait_lock, the unlocking code would spin on the new owner's pi_lock + but never give up the wait_lock. So the PI chain loop is guaranteed to + fail the spin_trylock on the wait_lock, release the pi_lock, and + try again. + + If you don't quite understand the above, that's OK. You don't have to, + unless you really want to make a proof out of it ;) + + +Pending Owners and Lock stealing +-------------------------------- + +One of the flags in the owner field of the mutex structure is "Pending Owner". +What this means is that an owner was chosen by the process releasing the +mutex, but that owner has yet to wake up and actually take the mutex. + +Why is this important? Why can't we just give the mutex to another process +and be done with it? + +The PI code is to help with real-time processes, and to let the highest +priority process run as long as possible with little latencies and delays. +If a high priority process owns a mutex that a lower priority process is +blocked on, when the mutex is released it would be given to the lower priority +process. What if the higher priority process wants to take that mutex again. +The high priority process would fail to take that mutex that it just gave up +and it would need to boost the lower priority process to run with full +latency of that critical section (since the low priority process just entered +it). + +There's no reason a high priority process that gives up a mutex should be +penalized if it tries to take that mutex again. If the new owner of the +mutex has not woken up yet, there's no reason that the higher priority process +could not take that mutex away. + +To solve this, we introduced Pending Ownership and Lock Stealing. When a +new process is given a mutex that it was blocked on, it is only given +pending ownership. This means that it's the new owner, unless a higher +priority process comes in and tries to grab that mutex. If a higher priority +process does come along and wants that mutex, we let the higher priority +process "steal" the mutex from the pending owner (only if it is still pending) +and continue with the mutex. + + +Taking of a mutex (The walk through) +------------------------------------ + +OK, now let's take a look at the detailed walk through of what happens when +taking a mutex. + +The first thing that is tried is the fast taking of the mutex. This is +done when we have CMPXCHG enabled (otherwise the fast taking automatically +fails). Only when the owner field of the mutex is NULL can the lock be +taken with the CMPXCHG and nothing else needs to be done. + +If there is contention on the lock, whether it is owned or pending owner +we go about the slow path (rt_mutex_slowlock). + +The slow path function is where the task's waiter structure is created on +the stack. This is because the waiter structure is only needed for the +scope of this function. The waiter structure holds the nodes to store +the task on the wait_list of the mutex, and if need be, the pi_list of +the owner. + +The wait_lock of the mutex is taken since the slow path of unlocking the +mutex also takes this lock. + +We then call try_to_take_rt_mutex. This is where the architecture that +does not implement CMPXCHG would always grab the lock (if there's no +contention). + +try_to_take_rt_mutex is used every time the task tries to grab a mutex in the +slow path. The first thing that is done here is an atomic setting of +the "Has Waiters" flag of the mutex's owner field. Yes, this could really +be false, because if the mutex has no owner, there are no waiters and +the current task also won't have any waiters. But we don't have the lock +yet, so we assume we are going to be a waiter. The reason for this is to +play nice for those architectures that do have CMPXCHG. By setting this flag +now, the owner of the mutex can't release the mutex without going into the +slow unlock path, and it would then need to grab the wait_lock, which this +code currently holds. So setting the "Has Waiters" flag forces the owner +to synchronize with this code. + +Now that we know that we can't have any races with the owner releasing the +mutex, we check to see if we can take the ownership. This is done if the +mutex doesn't have a owner, or if we can steal the mutex from a pending +owner. Let's look at the situations we have here. + + 1) Has owner that is pending + ---------------------------- + + The mutex has a owner, but it hasn't woken up and the mutex flag + "Pending Owner" is set. The first check is to see if the owner isn't the + current task. This is because this function is also used for the pending + owner to grab the mutex. When a pending owner wakes up, it checks to see + if it can take the mutex, and this is done if the owner is already set to + itself. If so, we succeed and leave the function, clearing the "Pending + Owner" bit. + + If the pending owner is not current, we check to see if the current priority is + higher than the pending owner. If not, we fail the function and return. + + There's also something special about a pending owner. That is a pending owner + is never blocked on a mutex. So there is no PI chain to worry about. It also + means that if the mutex doesn't have any waiters, there's no accounting needed + to update the pending owner's pi_list, since we only worry about processes + blocked on the current mutex. + + If there are waiters on this mutex, and we just stole the ownership, we need + to take the top waiter, remove it from the pi_list of the pending owner, and + add it to the current pi_list. Note that at this moment, the pending owner + is no longer on the list of waiters. This is fine, since the pending owner + would add itself back when it realizes that it had the ownership stolen + from itself. When the pending owner tries to grab the mutex, it will fail + in try_to_take_rt_mutex if the owner field points to another process. + + 2) No owner + ----------- + + If there is no owner (or we successfully stole the lock), we set the owner + of the mutex to current, and set the flag of "Has Waiters" if the current + mutex actually has waiters, or we clear the flag if it doesn't. See, it was + OK that we set that flag early, since now it is cleared. + + 3) Failed to grab ownership + --------------------------- + + The most interesting case is when we fail to take ownership. This means that + there exists an owner, or there's a pending owner with equal or higher + priority than the current task. + +We'll continue on the failed case. + +If the mutex has a timeout, we set up a timer to go off to break us out +of this mutex if we failed to get it after a specified amount of time. + +Now we enter a loop that will continue to try to take ownership of the mutex, or +fail from a timeout or signal. + +Once again we try to take the mutex. This will usually fail the first time +in the loop, since it had just failed to get the mutex. But the second time +in the loop, this would likely succeed, since the task would likely be +the pending owner. + +If the mutex is TASK_INTERRUPTIBLE a check for signals and timeout is done +here. + +The waiter structure has a "task" field that points to the task that is blocked +on the mutex. This field can be NULL the first time it goes through the loop +or if the task is a pending owner and had its mutex stolen. If the "task" +field is NULL then we need to set up the accounting for it. + +Task blocks on mutex +-------------------- + +The accounting of a mutex and process is done with the waiter structure of +the process. The "task" field is set to the process, and the "lock" field +to the mutex. The plist nodes are initialized to the processes current +priority. + +Since the wait_lock was taken at the entry of the slow lock, we can safely +add the waiter to the wait_list. If the current process is the highest +priority process currently waiting on this mutex, then we remove the +previous top waiter process (if it exists) from the pi_list of the owner, +and add the current process to that list. Since the pi_list of the owner +has changed, we call rt_mutex_adjust_prio on the owner to see if the owner +should adjust its priority accordingly. + +If the owner is also blocked on a lock, and had its pi_list changed +(or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead +and run rt_mutex_adjust_prio_chain on the owner, as described earlier. + +Now all locks are released, and if the current process is still blocked on a +mutex (waiter "task" field is not NULL), then we go to sleep (call schedule). + +Waking up in the loop +--------------------- + +The schedule can then wake up for a few reasons. + 1) we were given pending ownership of the mutex. + 2) we received a signal and was TASK_INTERRUPTIBLE + 3) we had a timeout and was TASK_INTERRUPTIBLE + +In any of these cases, we continue the loop and once again try to grab the +ownership of the mutex. If we succeed, we exit the loop, otherwise we continue +and on signal and timeout, will exit the loop, or if we had the mutex stolen +we just simply add ourselves back on the lists and go back to sleep. + +Note: For various reasons, because of timeout and signals, the steal mutex + algorithm needs to be careful. This is because the current process is + still on the wait_list. And because of dynamic changing of priorities, + especially on SCHED_OTHER tasks, the current process can be the + highest priority task on the wait_list. + +Failed to get mutex on Timeout or Signal +---------------------------------------- + +If a timeout or signal occurred, the waiter's "task" field would not be +NULL and the task needs to be taken off the wait_list of the mutex and perhaps +pi_list of the owner. If this process was a high priority process, then +the rt_mutex_adjust_prio_chain needs to be executed again on the owner, +but this time it will be lowering the priorities. + + +Unlocking the Mutex +------------------- + +The unlocking of a mutex also has a fast path for those architectures with +CMPXCHG. Since the taking of a mutex on contention always sets the +"Has Waiters" flag of the mutex's owner, we use this to know if we need to +take the slow path when unlocking the mutex. If the mutex doesn't have any +waiters, the owner field of the mutex would equal the current process and +the mutex can be unlocked by just replacing the owner field with NULL. + +If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available), +the slow unlock path is taken. + +The first thing done in the slow unlock path is to take the wait_lock of the +mutex. This synchronizes the locking and unlocking of the mutex. + +A check is made to see if the mutex has waiters or not. On architectures that +do not have CMPXCHG, this is the location that the owner of the mutex will +determine if a waiter needs to be awoken or not. On architectures that +do have CMPXCHG, that check is done in the fast path, but it is still needed +in the slow path too. If a waiter of a mutex woke up because of a signal +or timeout between the time the owner failed the fast path CMPXCHG check and +the grabbing of the wait_lock, the mutex may not have any waiters, thus the +owner still needs to make this check. If there are no waiters then the mutex +owner field is set to NULL, the wait_lock is released and nothing more is +needed. + +If there are waiters, then we need to wake one up and give that waiter +pending ownership. + +On the wake up code, the pi_lock of the current owner is taken. The top +waiter of the lock is found and removed from the wait_list of the mutex +as well as the pi_list of the current owner. The task field of the new +pending owner's waiter structure is set to NULL, and the owner field of the +mutex is set to the new owner with the "Pending Owner" bit set, as well +as the "Has Waiters" bit if there still are other processes blocked on the +mutex. + +The pi_lock of the previous owner is released, and the new pending owner's +pi_lock is taken. Remember that this is the trick to prevent the race +condition in rt_mutex_adjust_prio_chain from adding itself as a waiter +on the mutex. + +We now clear the "pi_blocked_on" field of the new pending owner, and if +the mutex still has waiters pending, we add the new top waiter to the pi_list +of the pending owner. + +Finally we unlock the pi_lock of the pending owner and wake it up. + + +Contact +------- + +For updates on this document, please email Steven Rostedt <rostedt@goodmis.org> + + +Credits +------- + +Author: Steven Rostedt <rostedt@goodmis.org> + +Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap + +Updates +------- + +This document was originally written for 2.6.17-rc3-mm1 diff --git a/Documentation/locking/rt-mutex.txt b/Documentation/locking/rt-mutex.txt new file mode 100644 index 000000000000..243393d882ee --- /dev/null +++ b/Documentation/locking/rt-mutex.txt @@ -0,0 +1,79 @@ +RT-mutex subsystem with PI support +---------------------------------- + +RT-mutexes with priority inheritance are used to support PI-futexes, +which enable pthread_mutex_t priority inheritance attributes +(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details +about PI-futexes.] + +This technology was developed in the -rt tree and streamlined for +pthread_mutex support. + +Basic principles: +----------------- + +RT-mutexes extend the semantics of simple mutexes by the priority +inheritance protocol. + +A low priority owner of a rt-mutex inherits the priority of a higher +priority waiter until the rt-mutex is released. If the temporarily +boosted owner blocks on a rt-mutex itself it propagates the priority +boosting to the owner of the other rt_mutex it gets blocked on. The +priority boosting is immediately removed once the rt_mutex has been +unlocked. + +This approach allows us to shorten the block of high-prio tasks on +mutexes which protect shared resources. Priority inheritance is not a +magic bullet for poorly designed applications, but it allows +well-designed applications to use userspace locks in critical parts of +an high priority thread, without losing determinism. + +The enqueueing of the waiters into the rtmutex waiter list is done in +priority order. For same priorities FIFO order is chosen. For each +rtmutex, only the top priority waiter is enqueued into the owner's +priority waiters list. This list too queues in priority order. Whenever +the top priority waiter of a task changes (for example it timed out or +got a signal), the priority of the owner task is readjusted. [The +priority enqueueing is handled by "plists", see include/linux/plist.h +for more details.] + +RT-mutexes are optimized for fastpath operations and have no internal +locking overhead when locking an uncontended mutex or unlocking a mutex +without waiters. The optimized fastpath operations require cmpxchg +support. [If that is not available then the rt-mutex internal spinlock +is used] + +The state of the rt-mutex is tracked via the owner field of the rt-mutex +structure: + +rt_mutex->owner holds the task_struct pointer of the owner. Bit 0 and 1 +are used to keep track of the "owner is pending" and "rtmutex has +waiters" state. + + owner bit1 bit0 + NULL 0 0 mutex is free (fast acquire possible) + NULL 0 1 invalid state + NULL 1 0 Transitional state* + NULL 1 1 invalid state + taskpointer 0 0 mutex is held (fast release possible) + taskpointer 0 1 task is pending owner + taskpointer 1 0 mutex is held and has waiters + taskpointer 1 1 task is pending owner and mutex has waiters + +Pending-ownership handling is a performance optimization: +pending-ownership is assigned to the first (highest priority) waiter of +the mutex, when the mutex is released. The thread is woken up and once +it starts executing it can acquire the mutex. Until the mutex is taken +by it (bit 0 is cleared) a competing higher priority thread can "steal" +the mutex which puts the woken up thread back on the waiters list. + +The pending-ownership optimization is especially important for the +uninterrupted workflow of high-prio tasks which repeatedly +takes/releases locks that have lower-prio waiters. Without this +optimization the higher-prio thread would ping-pong to the lower-prio +task [because at unlock time we always assign a new owner]. + +(*) The "mutex has waiters" bit gets set to take the lock. If the lock +doesn't already have an owner, this bit is quickly cleared if there are +no waiters. So this is a transitional state to synchronize with looking +at the owner field of the mutex and the mutex owner releasing the lock. diff --git a/Documentation/locking/spinlocks.txt b/Documentation/locking/spinlocks.txt new file mode 100644 index 000000000000..ff35e40bdf5b --- /dev/null +++ b/Documentation/locking/spinlocks.txt @@ -0,0 +1,167 @@ +Lesson 1: Spin locks + +The most basic primitive for locking is spinlock. + +static DEFINE_SPINLOCK(xxx_lock); + + unsigned long flags; + + spin_lock_irqsave(&xxx_lock, flags); + ... critical section here .. + spin_unlock_irqrestore(&xxx_lock, flags); + +The above is always safe. It will disable interrupts _locally_, but the +spinlock itself will guarantee the global lock, so it will guarantee that +there is only one thread-of-control within the region(s) protected by that +lock. This works well even under UP also, so the code does _not_ need to +worry about UP vs SMP issues: the spinlocks work correctly under both. + + NOTE! Implications of spin_locks for memory are further described in: + + Documentation/memory-barriers.txt + (5) LOCK operations. + (6) UNLOCK operations. + +The above is usually pretty simple (you usually need and want only one +spinlock for most things - using more than one spinlock can make things a +lot more complex and even slower and is usually worth it only for +sequences that you _know_ need to be split up: avoid it at all cost if you +aren't sure). + +This is really the only really hard part about spinlocks: once you start +using spinlocks they tend to expand to areas you might not have noticed +before, because you have to make sure the spinlocks correctly protect the +shared data structures _everywhere_ they are used. The spinlocks are most +easily added to places that are completely independent of other code (for +example, internal driver data structures that nobody else ever touches). + + NOTE! The spin-lock is safe only when you _also_ use the lock itself + to do locking across CPU's, which implies that EVERYTHING that + touches a shared variable has to agree about the spinlock they want + to use. + +---- + +Lesson 2: reader-writer spinlocks. + +If your data accesses have a very natural pattern where you usually tend +to mostly read from the shared variables, the reader-writer locks +(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple +readers to be in the same critical region at once, but if somebody wants +to change the variables it has to get an exclusive write lock. + + NOTE! reader-writer locks require more atomic memory operations than + simple spinlocks. Unless the reader critical section is long, you + are better off just using spinlocks. + +The routines look the same as above: + + rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock); + + unsigned long flags; + + read_lock_irqsave(&xxx_lock, flags); + .. critical section that only reads the info ... + read_unlock_irqrestore(&xxx_lock, flags); + + write_lock_irqsave(&xxx_lock, flags); + .. read and write exclusive access to the info ... + write_unlock_irqrestore(&xxx_lock, flags); + +The above kind of lock may be useful for complex data structures like +linked lists, especially searching for entries without changing the list +itself. The read lock allows many concurrent readers. Anything that +_changes_ the list will have to get the write lock. + + NOTE! RCU is better for list traversal, but requires careful + attention to design detail (see Documentation/RCU/listRCU.txt). + +Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_ +time need to do any changes (even if you don't do it every time), you have +to get the write-lock at the very beginning. + + NOTE! We are working hard to remove reader-writer spinlocks in most + cases, so please don't add a new one without consensus. (Instead, see + Documentation/RCU/rcu.txt for complete information.) + +---- + +Lesson 3: spinlocks revisited. + +The single spin-lock primitives above are by no means the only ones. They +are the most safe ones, and the ones that work under all circumstances, +but partly _because_ they are safe they are also fairly slow. They are slower +than they'd need to be, because they do have to disable interrupts +(which is just a single instruction on a x86, but it's an expensive one - +and on other architectures it can be worse). + +If you have a case where you have to protect a data structure across +several CPU's and you want to use spinlocks you can potentially use +cheaper versions of the spinlocks. IFF you know that the spinlocks are +never used in interrupt handlers, you can use the non-irq versions: + + spin_lock(&lock); + ... + spin_unlock(&lock); + +(and the equivalent read-write versions too, of course). The spinlock will +guarantee the same kind of exclusive access, and it will be much faster. +This is useful if you know that the data in question is only ever +manipulated from a "process context", ie no interrupts involved. + +The reasons you mustn't use these versions if you have interrupts that +play with the spinlock is that you can get deadlocks: + + spin_lock(&lock); + ... + <- interrupt comes in: + spin_lock(&lock); + +where an interrupt tries to lock an already locked variable. This is ok if +the other interrupt happens on another CPU, but it is _not_ ok if the +interrupt happens on the same CPU that already holds the lock, because the +lock will obviously never be released (because the interrupt is waiting +for the lock, and the lock-holder is interrupted by the interrupt and will +not continue until the interrupt has been processed). + +(This is also the reason why the irq-versions of the spinlocks only need +to disable the _local_ interrupts - it's ok to use spinlocks in interrupts +on other CPU's, because an interrupt on another CPU doesn't interrupt the +CPU that holds the lock, so the lock-holder can continue and eventually +releases the lock). + +Note that you can be clever with read-write locks and interrupts. For +example, if you know that the interrupt only ever gets a read-lock, then +you can use a non-irq version of read locks everywhere - because they +don't block on each other (and thus there is no dead-lock wrt interrupts. +But when you do the write-lock, you have to use the irq-safe version. + +For an example of being clever with rw-locks, see the "waitqueue_lock" +handling in kernel/sched/core.c - nothing ever _changes_ a wait-queue from +within an interrupt, they only read the queue in order to know whom to +wake up. So read-locks are safe (which is good: they are very common +indeed), while write-locks need to protect themselves against interrupts. + + Linus + +---- + +Reference information: + +For dynamic initialization, use spin_lock_init() or rwlock_init() as +appropriate: + + spinlock_t xxx_lock; + rwlock_t xxx_rw_lock; + + static int __init xxx_init(void) + { + spin_lock_init(&xxx_lock); + rwlock_init(&xxx_rw_lock); + ... + } + + module_init(xxx_init); + +For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or +__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate. diff --git a/Documentation/locking/ww-mutex-design.txt b/Documentation/locking/ww-mutex-design.txt new file mode 100644 index 000000000000..8a112dc304c3 --- /dev/null +++ b/Documentation/locking/ww-mutex-design.txt @@ -0,0 +1,344 @@ +Wait/Wound Deadlock-Proof Mutex Design +====================================== + +Please read mutex-design.txt first, as it applies to wait/wound mutexes too. + +Motivation for WW-Mutexes +------------------------- + +GPU's do operations that commonly involve many buffers. Those buffers +can be shared across contexts/processes, exist in different memory +domains (for example VRAM vs system memory), and so on. And with +PRIME / dmabuf, they can even be shared across devices. So there are +a handful of situations where the driver needs to wait for buffers to +become ready. If you think about this in terms of waiting on a buffer +mutex for it to become available, this presents a problem because +there is no way to guarantee that buffers appear in a execbuf/batch in +the same order in all contexts. That is directly under control of +userspace, and a result of the sequence of GL calls that an application +makes. Which results in the potential for deadlock. The problem gets +more complex when you consider that the kernel may need to migrate the +buffer(s) into VRAM before the GPU operates on the buffer(s), which +may in turn require evicting some other buffers (and you don't want to +evict other buffers which are already queued up to the GPU), but for a +simplified understanding of the problem you can ignore this. + +The algorithm that the TTM graphics subsystem came up with for dealing with +this problem is quite simple. For each group of buffers (execbuf) that need +to be locked, the caller would be assigned a unique reservation id/ticket, +from a global counter. In case of deadlock while locking all the buffers +associated with a execbuf, the one with the lowest reservation ticket (i.e. +the oldest task) wins, and the one with the higher reservation id (i.e. the +younger task) unlocks all of the buffers that it has already locked, and then +tries again. + +In the RDBMS literature this deadlock handling approach is called wait/wound: +The older tasks waits until it can acquire the contended lock. The younger tasks +needs to back off and drop all the locks it is currently holding, i.e. the +younger task is wounded. + +Concepts +-------- + +Compared to normal mutexes two additional concepts/objects show up in the lock +interface for w/w mutexes: + +Acquire context: To ensure eventual forward progress it is important the a task +trying to acquire locks doesn't grab a new reservation id, but keeps the one it +acquired when starting the lock acquisition. This ticket is stored in the +acquire context. Furthermore the acquire context keeps track of debugging state +to catch w/w mutex interface abuse. + +W/w class: In contrast to normal mutexes the lock class needs to be explicit for +w/w mutexes, since it is required to initialize the acquire context. + +Furthermore there are three different class of w/w lock acquire functions: + +* Normal lock acquisition with a context, using ww_mutex_lock. + +* Slowpath lock acquisition on the contending lock, used by the wounded task + after having dropped all already acquired locks. These functions have the + _slow postfix. + + From a simple semantics point-of-view the _slow functions are not strictly + required, since simply calling the normal ww_mutex_lock functions on the + contending lock (after having dropped all other already acquired locks) will + work correctly. After all if no other ww mutex has been acquired yet there's + no deadlock potential and hence the ww_mutex_lock call will block and not + prematurely return -EDEADLK. The advantage of the _slow functions is in + interface safety: + - ww_mutex_lock has a __must_check int return type, whereas ww_mutex_lock_slow + has a void return type. Note that since ww mutex code needs loops/retries + anyway the __must_check doesn't result in spurious warnings, even though the + very first lock operation can never fail. + - When full debugging is enabled ww_mutex_lock_slow checks that all acquired + ww mutex have been released (preventing deadlocks) and makes sure that we + block on the contending lock (preventing spinning through the -EDEADLK + slowpath until the contended lock can be acquired). + +* Functions to only acquire a single w/w mutex, which results in the exact same + semantics as a normal mutex. This is done by calling ww_mutex_lock with a NULL + context. + + Again this is not strictly required. But often you only want to acquire a + single lock in which case it's pointless to set up an acquire context (and so + better to avoid grabbing a deadlock avoidance ticket). + +Of course, all the usual variants for handling wake-ups due to signals are also +provided. + +Usage +----- + +Three different ways to acquire locks within the same w/w class. Common +definitions for methods #1 and #2: + +static DEFINE_WW_CLASS(ww_class); + +struct obj { + struct ww_mutex lock; + /* obj data */ +}; + +struct obj_entry { + struct list_head head; + struct obj *obj; +}; + +Method 1, using a list in execbuf->buffers that's not allowed to be reordered. +This is useful if a list of required objects is already tracked somewhere. +Furthermore the lock helper can use propagate the -EALREADY return code back to +the caller as a signal that an object is twice on the list. This is useful if +the list is constructed from userspace input and the ABI requires userspace to +not have duplicate entries (e.g. for a gpu commandbuffer submission ioctl). + +int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) +{ + struct obj *res_obj = NULL; + struct obj_entry *contended_entry = NULL; + struct obj_entry *entry; + + ww_acquire_init(ctx, &ww_class); + +retry: + list_for_each_entry (entry, list, head) { + if (entry->obj == res_obj) { + res_obj = NULL; + continue; + } + ret = ww_mutex_lock(&entry->obj->lock, ctx); + if (ret < 0) { + contended_entry = entry; + goto err; + } + } + + ww_acquire_done(ctx); + return 0; + +err: + list_for_each_entry_continue_reverse (entry, list, head) + ww_mutex_unlock(&entry->obj->lock); + + if (res_obj) + ww_mutex_unlock(&res_obj->lock); + + if (ret == -EDEADLK) { + /* we lost out in a seqno race, lock and retry.. */ + ww_mutex_lock_slow(&contended_entry->obj->lock, ctx); + res_obj = contended_entry->obj; + goto retry; + } + ww_acquire_fini(ctx); + + return ret; +} + +Method 2, using a list in execbuf->buffers that can be reordered. Same semantics +of duplicate entry detection using -EALREADY as method 1 above. But the +list-reordering allows for a bit more idiomatic code. + +int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) +{ + struct obj_entry *entry, *entry2; + + ww_acquire_init(ctx, &ww_class); + + list_for_each_entry (entry, list, head) { + ret = ww_mutex_lock(&entry->obj->lock, ctx); + if (ret < 0) { + entry2 = entry; + + list_for_each_entry_continue_reverse (entry2, list, head) + ww_mutex_unlock(&entry2->obj->lock); + + if (ret != -EDEADLK) { + ww_acquire_fini(ctx); + return ret; + } + + /* we lost out in a seqno race, lock and retry.. */ + ww_mutex_lock_slow(&entry->obj->lock, ctx); + + /* + * Move buf to head of the list, this will point + * buf->next to the first unlocked entry, + * restarting the for loop. + */ + list_del(&entry->head); + list_add(&entry->head, list); + } + } + + ww_acquire_done(ctx); + return 0; +} + +Unlocking works the same way for both methods #1 and #2: + +void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) +{ + struct obj_entry *entry; + + list_for_each_entry (entry, list, head) + ww_mutex_unlock(&entry->obj->lock); + + ww_acquire_fini(ctx); +} + +Method 3 is useful if the list of objects is constructed ad-hoc and not upfront, +e.g. when adjusting edges in a graph where each node has its own ww_mutex lock, +and edges can only be changed when holding the locks of all involved nodes. w/w +mutexes are a natural fit for such a case for two reasons: +- They can handle lock-acquisition in any order which allows us to start walking + a graph from a starting point and then iteratively discovering new edges and + locking down the nodes those edges connect to. +- Due to the -EALREADY return code signalling that a given objects is already + held there's no need for additional book-keeping to break cycles in the graph + or keep track off which looks are already held (when using more than one node + as a starting point). + +Note that this approach differs in two important ways from the above methods: +- Since the list of objects is dynamically constructed (and might very well be + different when retrying due to hitting the -EDEADLK wound condition) there's + no need to keep any object on a persistent list when it's not locked. We can + therefore move the list_head into the object itself. +- On the other hand the dynamic object list construction also means that the -EALREADY return + code can't be propagated. + +Note also that methods #1 and #2 and method #3 can be combined, e.g. to first lock a +list of starting nodes (passed in from userspace) using one of the above +methods. And then lock any additional objects affected by the operations using +method #3 below. The backoff/retry procedure will be a bit more involved, since +when the dynamic locking step hits -EDEADLK we also need to unlock all the +objects acquired with the fixed list. But the w/w mutex debug checks will catch +any interface misuse for these cases. + +Also, method 3 can't fail the lock acquisition step since it doesn't return +-EALREADY. Of course this would be different when using the _interruptible +variants, but that's outside of the scope of these examples here. + +struct obj { + struct ww_mutex ww_mutex; + struct list_head locked_list; +}; + +static DEFINE_WW_CLASS(ww_class); + +void __unlock_objs(struct list_head *list) +{ + struct obj *entry, *temp; + + list_for_each_entry_safe (entry, temp, list, locked_list) { + /* need to do that before unlocking, since only the current lock holder is + allowed to use object */ + list_del(&entry->locked_list); + ww_mutex_unlock(entry->ww_mutex) + } +} + +void lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) +{ + struct obj *obj; + + ww_acquire_init(ctx, &ww_class); + +retry: + /* re-init loop start state */ + loop { + /* magic code which walks over a graph and decides which objects + * to lock */ + + ret = ww_mutex_lock(obj->ww_mutex, ctx); + if (ret == -EALREADY) { + /* we have that one already, get to the next object */ + continue; + } + if (ret == -EDEADLK) { + __unlock_objs(list); + + ww_mutex_lock_slow(obj, ctx); + list_add(&entry->locked_list, list); + goto retry; + } + + /* locked a new object, add it to the list */ + list_add_tail(&entry->locked_list, list); + } + + ww_acquire_done(ctx); + return 0; +} + +void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx) +{ + __unlock_objs(list); + ww_acquire_fini(ctx); +} + +Method 4: Only lock one single objects. In that case deadlock detection and +prevention is obviously overkill, since with grabbing just one lock you can't +produce a deadlock within just one class. To simplify this case the w/w mutex +api can be used with a NULL context. + +Implementation Details +---------------------- + +Design: + ww_mutex currently encapsulates a struct mutex, this means no extra overhead for + normal mutex locks, which are far more common. As such there is only a small + increase in code size if wait/wound mutexes are not used. + + In general, not much contention is expected. The locks are typically used to + serialize access to resources for devices. The only way to make wakeups + smarter would be at the cost of adding a field to struct mutex_waiter. This + would add overhead to all cases where normal mutexes are used, and + ww_mutexes are generally less performance sensitive. + +Lockdep: + Special care has been taken to warn for as many cases of api abuse + as possible. Some common api abuses will be caught with + CONFIG_DEBUG_MUTEXES, but CONFIG_PROVE_LOCKING is recommended. + + Some of the errors which will be warned about: + - Forgetting to call ww_acquire_fini or ww_acquire_init. + - Attempting to lock more mutexes after ww_acquire_done. + - Attempting to lock the wrong mutex after -EDEADLK and + unlocking all mutexes. + - Attempting to lock the right mutex after -EDEADLK, + before unlocking all mutexes. + + - Calling ww_mutex_lock_slow before -EDEADLK was returned. + + - Unlocking mutexes with the wrong unlock function. + - Calling one of the ww_acquire_* twice on the same context. + - Using a different ww_class for the mutex than for the ww_acquire_ctx. + - Normal lockdep errors that can result in deadlocks. + + Some of the lockdep errors that can result in deadlocks: + - Calling ww_acquire_init to initialize a second ww_acquire_ctx before + having called ww_acquire_fini on the first. + - 'normal' deadlocks that can occur. + +FIXME: Update this section once we have the TASK_DEADLOCK task state flag magic +implemented. |