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| author | Jason Low <jason.low2@hp.com> | 2014-06-11 11:37:22 -0700 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2014-07-05 11:25:42 +0200 |
| commit | 0d968dd8c6aced585b86fa7ba8ce4573bf19e848 (patch) | |
| tree | 08d135faf746fc75290f3f16c71eb651b6fbe948 /kernel/locking | |
| parent | 1e820c9608eace237e2c519d8fd9074aec479d81 (diff) | |
| download | linux-0d968dd8c6aced585b86fa7ba8ce4573bf19e848.tar.gz linux-0d968dd8c6aced585b86fa7ba8ce4573bf19e848.tar.bz2 linux-0d968dd8c6aced585b86fa7ba8ce4573bf19e848.zip | |
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>
Diffstat (limited to 'kernel/locking')
| -rw-r--r-- | kernel/locking/mutex.c | 7 |
1 files changed, 4 insertions, 3 deletions
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 4bd95465af55..e4d997bb7d70 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); |