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authorJason Low <jason.low2@hp.com>2014-06-11 11:37:23 -0700
committerIngo Molnar <mingo@kernel.org>2014-07-05 11:25:42 +0200
commit72d5305dcb3637913c2c37e847a4de9028e49244 (patch)
treef0e51c5802529136b5d16861f7caf50cac6cc906
parent0d968dd8c6aced585b86fa7ba8ce4573bf19e848 (diff)
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locking/mutexes: Optimize mutex trylock slowpath
The mutex_trylock() function calls into __mutex_trylock_fastpath() when trying to obtain the mutex. On 32 bit x86, in the !__HAVE_ARCH_CMPXCHG case, __mutex_trylock_fastpath() calls directly into __mutex_trylock_slowpath() regardless of whether or not the mutex is locked. In __mutex_trylock_slowpath(), we then acquire the wait_lock spinlock, xchg() lock->count with -1, then set lock->count back to 0 if there are no waiters, and return true if the prev lock count was 1. However, if the mutex is already locked, then there isn't much point in attempting all of the above expensive operations. In this patch, we only attempt the above trylock operations if the mutex is unlocked. Signed-off-by: Jason Low <jason.low2@hp.com> Reviewed-by: Davidlohr Bueso <davidlohr@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: Waiman.Long@hp.com Cc: scott.norton@hp.com Cc: aswin@hp.com Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1402511843-4721-5-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--kernel/locking/mutex.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index e4d997b..11b103d 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -820,6 +820,10 @@ static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
unsigned long flags;
int prev;
+ /* No need to trylock if the mutex is locked. */
+ if (mutex_is_locked(lock))
+ return 0;
+
spin_lock_mutex(&lock->wait_lock, flags);
prev = atomic_xchg(&lock->count, -1);
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