Messages in this thread Patch in this message | | | Date | Sat, 5 Jul 2014 03:47:50 -0700 | From | tip-bot for Jason Low <> | Subject | [tip:locking/core] locking/mutexes: Try to acquire mutex only if it is unlocked |
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Commit-ID: 0d968dd8c6aced585b86fa7ba8ce4573bf19e848 Gitweb: http://git.kernel.org/tip/0d968dd8c6aced585b86fa7ba8ce4573bf19e848 Author: Jason Low <jason.low2@hp.com> AuthorDate: Wed, 11 Jun 2014 11:37:22 -0700 Committer: Ingo Molnar <mingo@kernel.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@hp.com> Acked-by: Waiman Long <Waiman.Long@hp.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: akpm@linux-foundation.org Cc: tim.c.chen@linux.intel.com Cc: paulmck@linux.vnet.ibm.com Cc: rostedt@goodmis.org Cc: davidlohr@hp.com Cc: scott.norton@hp.com Cc: aswin@hp.com Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1402511843-4721-4-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.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);
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