Upon entering the slowpath in __mutex_lock_common(), we try once more to
acquire the mutex 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 to further reduce unnecessary 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.
Also, delete the MUTEX_SHOW_NO_WAITER() macro, because the macro doesn't
always do what its name suggests. Directly use atomic_read() instead of
the macro and add comments on how the extra atomic_read() can be helpful.
Acked-by: Waiman Long <waiman.l...@hp.com>
Acked-by: Davidlohr Bueso <davidl...@hp.com>
Signed-off-by: Jason Low <jason.l...@hp.com>
---
kernel/locking/mutex.c | 21 ++++++++++-----------
1 files changed, 10 insertions(+), 11 deletions(-)
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index dd26bf6..e4d997b 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -46,12 +46,6 @@
# include <asm/mutex.h>
#endif
-/*
- * A negative mutex count indicates that waiters are sleeping waiting for the
- * mutex.
- */
-#define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count)
>= 0)
-
void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
@@ -438,7 +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) {
@@ -483,8 +478,11 @@ slowpath:
#endif
spin_lock_mutex(&lock->wait_lock, flags);
- /* once more, can we acquire the lock? */
- if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, 0) == 1))
+ /*
+ * Once more, try to acquire the lock. Only try-lock the mutex if
+ * it is unlocked to reduce unnecessary xchg() operations.
+ */
+ if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1))
goto skip_wait;
debug_mutex_lock_common(lock, &waiter);
@@ -504,9 +502,10 @@ slowpath:
* it's unlocked. Later on, if we sleep, this is the
* operation that gives us the lock. We xchg it to -1, so
* that when we release the lock, we properly wake up the
- * other waiters:
+ * other waiters. We only attempt the xchg if the count is
+ * non-negative in order to avoid unnecessary xchg operations:
*/
- if (MUTEX_SHOW_NO_WAITER(lock) &&
+ if (atomic_read(&lock->count) >= 0 &&
(atomic_xchg(&lock->count, -1) == 1))
break;
--
1.7.1
--
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