Commit 72d5305d authored by Jason Low's avatar Jason Low Committed by Ingo Molnar

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: default avatarJason Low <jason.low2@hp.com>
Reviewed-by: default avatarDavidlohr Bueso <davidlohr@hp.com>
Signed-off-by: default avatarPeter 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.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 0d968dd8
......@@ -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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