diff options
| -rw-r--r-- | Documentation/locking/lockdep-design.txt | 79 |
1 files changed, 61 insertions, 18 deletions
diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt index 39fae143c9cb..ae65758383ea 100644 --- a/Documentation/locking/lockdep-design.txt +++ b/Documentation/locking/lockdep-design.txt @@ -15,34 +15,48 @@ 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. +The validator tracks the 'usage state' of lock-classes, and it tracks +the dependencies between different lock-classes. Lock usage indicates +how a lock is used with regard to its IRQ contexts, while lock +dependency can be understood as lock order, where L1 -> L2 suggests that +a task is attempting to acquire L2 while holding L1. From lockdep's +perspective, the two locks (L1 and L2) are not necessarily related; that +dependency just means the order ever happened. The validator maintains a +continuing effort to prove lock usages and dependencies are correct or +the validator will shoot a splat if incorrect. + +A lock-class's behavior is constructed by its instances collectively: +when the first instance of a lock-class is used after bootup the class +gets registered, then all (subsequent) instances will be mapped to the +class and hence their usages and dependecies will contribute to those of +the class. A lock-class does not go away when a lock instance does, but +it can be removed if the memory space of the lock class (static or +dynamic) is reclaimed, this happens for example when a module is +unloaded or a workqueue is destroyed. State ----- -The validator tracks lock-class usage history into 4 * nSTATEs + 1 separate -state bits: +The validator tracks lock-class usage history and divides the usage into +(4 usages * n STATEs + 1) categories: +where the 4 usages can be: - '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/locking/lockdep_states.h) - - hardirq - - softirq +where the n STATEs are coded in kernel/locking/lockdep_states.h and as of +now they include: +- hardirq +- softirq +where the last 1 category is: - 'ever used' [ == !unused ] -When locking rules are violated, these state bits are presented in the -locking error messages, inside curlies. A contrived example: +When locking rules are violated, these usage bits are presented in the +locking error messages, inside curlies, with a total of 2 * n STATEs bits. +A contrived example: modprobe/2287 is trying to acquire lock: (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24 @@ -51,15 +65,44 @@ locking error messages, inside curlies. A contrived example: (&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: +For a given lock, the bit positions from left to right indicate the usage +of the lock and readlock (if exists), for each of the n STATEs listed +above respectively, and the character displayed at each bit position +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. +The bits are illustrated with an example: + + (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24 + |||| + ||| \-> softirq disabled and not in softirq context + || \--> acquired in softirq context + | \---> hardirq disabled and not in hardirq context + \----> acquired in hardirq context + + +For a given STATE, whether the lock is ever acquired in that STATE +context and whether that STATE is enabled yields four possible cases as +shown in the table below. The bit character is able to indicate which +exact case is for the lock as of the reporting time. + + ------------------------------------------- + | | irq enabled | irq disabled | + |-------------------------------------------| + | ever in irq | ? | - | + |-------------------------------------------| + | never in irq | + | . | + ------------------------------------------- + +The character '-' suggests irq is disabled because if otherwise the +charactor '?' would have been shown instead. Similar deduction can be +applied for '+' too. + +Unused locks (e.g., mutexes) cannot be part of the cause of an error. Single-lock state rules: |