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>

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 4bd95465af55fec305add8bf896b67f1222dbf8f..e4d997bb7d70acd1009e4eadc0368f9670a833e3 100644 (file)
--- 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);