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Date: Sat, 5 Jul 2014 03:47:50 -0700 From: tip-bot for Jason Low <tipbot@...or.com> To: linux-tip-commits@...r.kernel.org Cc: linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org, torvalds@...ux-foundation.org, peterz@...radead.org, jason.low2@...com, Waiman.Long@...com, tglx@...utronix.de Subject: [tip:locking/core] locking/mutexes: Try to acquire mutex only if it is unlocked Commit-ID: 0d968dd8c6aced585b86fa7ba8ce4573bf19e848 Gitweb: http://git.kernel.org/tip/0d968dd8c6aced585b86fa7ba8ce4573bf19e848 Author: Jason Low <jason.low2@...com> AuthorDate: Wed, 11 Jun 2014 11:37:22 -0700 Committer: Ingo Molnar <mingo@...nel.org> CommitDate: Sat, 5 Jul 2014 11:25:42 +0200 locking/mutexes: Try to acquire mutex only if it is unlocked Upon entering the slowpath in __mutex_lock_common(), we try once more to acquire the mutex. We only try to acquire if (lock->count >= 0). However, what we actually want here is to try to acquire if the mutex is unlocked (lock->count == 1). This patch changes it so that we only try-acquire the mutex upon entering the slowpath if it is unlocked, rather than if the lock count is non-negative. This helps further reduce unnecessary atomic xchg() operations. Furthermore, this patch uses !mutex_is_locked(lock) to do the initial checks for if the lock is free rather than directly calling atomic_read() on the lock->count, in order to improve readability. Signed-off-by: Jason Low <jason.low2@...com> Acked-by: Waiman Long <Waiman.Long@...com> Signed-off-by: Peter Zijlstra <peterz@...radead.org> Cc: akpm@...ux-foundation.org Cc: tim.c.chen@...ux.intel.com Cc: paulmck@...ux.vnet.ibm.com Cc: rostedt@...dmis.org Cc: davidlohr@...com Cc: scott.norton@...com Cc: aswin@...com Cc: Linus Torvalds <torvalds@...ux-foundation.org> Link: http://lkml.kernel.org/r/1402511843-4721-4-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar <mingo@...nel.org> --- kernel/locking/mutex.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4bd9546..e4d997b 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -432,7 +432,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, if (owner && !mutex_spin_on_owner(lock, owner)) break; - if ((atomic_read(&lock->count) == 1) && + /* Try to acquire the mutex if it is unlocked. */ + if (!mutex_is_locked(lock) && (atomic_cmpxchg(&lock->count, 1, 0) == 1)) { lock_acquired(&lock->dep_map, ip); if (use_ww_ctx) { @@ -479,9 +480,9 @@ slowpath: /* * Once more, try to acquire the lock. Only try-lock the mutex if - * lock->count >= 0 to reduce unnecessary xchg operations. + * it is unlocked to reduce unnecessary xchg() operations. */ - if (atomic_read(&lock->count) >= 0 && (atomic_xchg(&lock->count, 0) == 1)) + if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1)) goto skip_wait; debug_mutex_lock_common(lock, &waiter); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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