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author | Davidlohr Bueso <dave@stgolabs.net> | 2015-01-06 11:45:07 -0800 |
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committer | Ingo Molnar <mingo@kernel.org> | 2015-01-14 15:07:32 +0100 |
commit | d84b6728c54dcf73bcef3e3f7cf6767e2d224e39 (patch) | |
tree | c3d8f60aee0a71f79666e3d47df73deff975a5ab /kernel/locking/mcs_spinlock.c | |
parent | 4bd19084faa61a8c68586e74f03f5776179f65c2 (diff) | |
download | linux-d84b6728c54dcf73bcef3e3f7cf6767e2d224e39.tar.bz2 |
locking/mcs: Better differentiate between MCS variants
We have two flavors of the MCS spinlock: standard and cancelable (OSQ).
While each one is independent of the other, we currently mix and match
them. This patch:
- Moves the OSQ code out of mcs_spinlock.h (which only deals with the traditional
version) into include/linux/osq_lock.h. No unnecessary code is added to the
more global header file, anything locks that make use of OSQ must include
it anyway.
- Renames mcs_spinlock.c to osq_lock.c. This file only contains osq code.
- Introduces a CONFIG_LOCK_SPIN_ON_OWNER in order to only build osq_lock
if there is support for it.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Link: http://lkml.kernel.org/r/1420573509-24774-5-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/locking/mcs_spinlock.c')
-rw-r--r-- | kernel/locking/mcs_spinlock.c | 208 |
1 files changed, 0 insertions, 208 deletions
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c deleted file mode 100644 index 9887a905a762..000000000000 --- a/kernel/locking/mcs_spinlock.c +++ /dev/null @@ -1,208 +0,0 @@ -#include <linux/percpu.h> -#include <linux/sched.h> -#include "mcs_spinlock.h" - -#ifdef CONFIG_SMP - -/* - * An MCS like lock especially tailored for optimistic spinning for sleeping - * lock implementations (mutex, rwsem, etc). - * - * Using a single mcs node per CPU is safe because sleeping locks should not be - * called from interrupt context and we have preemption disabled while - * spinning. - */ -static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); - -/* - * We use the value 0 to represent "no CPU", thus the encoded value - * will be the CPU number incremented by 1. - */ -static inline int encode_cpu(int cpu_nr) -{ - return cpu_nr + 1; -} - -static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) -{ - int cpu_nr = encoded_cpu_val - 1; - - return per_cpu_ptr(&osq_node, cpu_nr); -} - -/* - * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. - * Can return NULL in case we were the last queued and we updated @lock instead. - */ -static inline struct optimistic_spin_node * -osq_wait_next(struct optimistic_spin_queue *lock, - struct optimistic_spin_node *node, - struct optimistic_spin_node *prev) -{ - struct optimistic_spin_node *next = NULL; - int curr = encode_cpu(smp_processor_id()); - int old; - - /* - * If there is a prev node in queue, then the 'old' value will be - * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if - * we're currently last in queue, then the queue will then become empty. - */ - old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; - - for (;;) { - if (atomic_read(&lock->tail) == curr && - atomic_cmpxchg(&lock->tail, curr, old) == curr) { - /* - * We were the last queued, we moved @lock back. @prev - * will now observe @lock and will complete its - * unlock()/unqueue(). - */ - break; - } - - /* - * We must xchg() the @node->next value, because if we were to - * leave it in, a concurrent unlock()/unqueue() from - * @node->next might complete Step-A and think its @prev is - * still valid. - * - * If the concurrent unlock()/unqueue() wins the race, we'll - * wait for either @lock to point to us, through its Step-B, or - * wait for a new @node->next from its Step-C. - */ - if (node->next) { - next = xchg(&node->next, NULL); - if (next) - break; - } - - cpu_relax_lowlatency(); - } - - return next; -} - -bool osq_lock(struct optimistic_spin_queue *lock) -{ - struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); - struct optimistic_spin_node *prev, *next; - int curr = encode_cpu(smp_processor_id()); - int old; - - node->locked = 0; - node->next = NULL; - node->cpu = curr; - - old = atomic_xchg(&lock->tail, curr); - if (old == OSQ_UNLOCKED_VAL) - return true; - - prev = decode_cpu(old); - node->prev = prev; - ACCESS_ONCE(prev->next) = node; - - /* - * Normally @prev is untouchable after the above store; because at that - * moment unlock can proceed and wipe the node element from stack. - * - * However, since our nodes are static per-cpu storage, we're - * guaranteed their existence -- this allows us to apply - * cmpxchg in an attempt to undo our queueing. - */ - - while (!smp_load_acquire(&node->locked)) { - /* - * If we need to reschedule bail... so we can block. - */ - if (need_resched()) - goto unqueue; - - cpu_relax_lowlatency(); - } - return true; - -unqueue: - /* - * Step - A -- stabilize @prev - * - * Undo our @prev->next assignment; this will make @prev's - * unlock()/unqueue() wait for a next pointer since @lock points to us - * (or later). - */ - - for (;;) { - if (prev->next == node && - cmpxchg(&prev->next, node, NULL) == node) - break; - - /* - * We can only fail the cmpxchg() racing against an unlock(), - * in which case we should observe @node->locked becomming - * true. - */ - if (smp_load_acquire(&node->locked)) - return true; - - cpu_relax_lowlatency(); - - /* - * Or we race against a concurrent unqueue()'s step-B, in which - * case its step-C will write us a new @node->prev pointer. - */ - prev = ACCESS_ONCE(node->prev); - } - - /* - * Step - B -- stabilize @next - * - * Similar to unlock(), wait for @node->next or move @lock from @node - * back to @prev. - */ - - next = osq_wait_next(lock, node, prev); - if (!next) - return false; - - /* - * Step - C -- unlink - * - * @prev is stable because its still waiting for a new @prev->next - * pointer, @next is stable because our @node->next pointer is NULL and - * it will wait in Step-A. - */ - - ACCESS_ONCE(next->prev) = prev; - ACCESS_ONCE(prev->next) = next; - - return false; -} - -void osq_unlock(struct optimistic_spin_queue *lock) -{ - struct optimistic_spin_node *node, *next; - int curr = encode_cpu(smp_processor_id()); - - /* - * Fast path for the uncontended case. - */ - if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr)) - return; - - /* - * Second most likely case. - */ - node = this_cpu_ptr(&osq_node); - next = xchg(&node->next, NULL); - if (next) { - ACCESS_ONCE(next->locked) = 1; - return; - } - - next = osq_wait_next(lock, node, NULL); - if (next) - ACCESS_ONCE(next->locked) = 1; -} - -#endif - |