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authorJason Low <jason.low2@hp.com>2014-06-11 11:37:21 -0700
committerIngo Molnar <mingo@kernel.org>2014-07-05 11:25:41 +0200
commit1e820c9608eace237e2c519d8fd9074aec479d81 (patch)
tree5d48b95b5b18110164bb9133e027fcc8d6872fbb /kernel/locking
parent0c3c0f0d6e56422cef60a33726d062e9923005c3 (diff)
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locking/mutexes: Delete the MUTEX_SHOW_NO_WAITER macro
MUTEX_SHOW_NO_WAITER() is a macro which checks for if there are "no waiters" on a mutex by checking if the lock count is non-negative. Based on feedback from the discussion in the earlier version of this patchset, the macro is not very readable. Furthermore, checking lock->count isn't always the correct way to determine if there are "no waiters" on a mutex. For example, a negative count on a mutex really only means that there "potentially" are waiters. Likewise, there can be waiters on the mutex even if the count is non-negative. Thus, "MUTEX_SHOW_NO_WAITER" doesn't always do what the name of the macro suggests. So this patch deletes the MUTEX_SHOW_NO_WAITERS() macro, directly use atomic_read() instead of the macro, and adds comments which elaborate on how the extra atomic_read() checks can help reduce unnecessary xchg() operations. 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/1402511843-4721-3-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.c18
1 files changed, 8 insertions, 10 deletions
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index dd26bf6dee0c..4bd95465af55 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)
{
@@ -483,8 +477,11 @@ 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
+ * lock->count >= 0 to reduce unnecessary xchg operations.
+ */
+ if (atomic_read(&lock->count) >= 0 && (atomic_xchg(&lock->count, 0) == 1))
goto skip_wait;
debug_mutex_lock_common(lock, &waiter);
@@ -504,9 +501,10 @@ 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;