Re: [PATCH v2 2/4] mutex: Delete the MUTEX_SHOW_NO_WAITER macro

From: Long, Wai Man
Date: Wed Jun 11 2014 - 21:27:44 EST



On 6/11/2014 2:37 PM, Jason Low wrote:
v1->v2:
- There were discussions in v1 about a possible mutex_has_waiters()
function. This patch didn't use that function because the places which
used MUTEX_SHOW_NO_WAITER requires checking for lock->count while an
actual mutex_has_waiters() should check for !list_empty(wait_list).
We'll just delete the macro and directly use atomic_read() + comments.

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@xxxxxx>
---
kernel/locking/mutex.c | 18 ++++++++----------
1 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index dd26bf6..4bd9546 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;

Acked-by: Waiman Long <Waiman.Long@xxxxxx>

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