From 1e820c9608eace237e2c519d8fd9074aec479d81 Mon Sep 17 00:00:00 2001 From: Jason Low Date: Wed, 11 Jun 2014 11:37:21 -0700 Subject: [PATCH] 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 Acked-by: Waiman Long Signed-off-by: Peter Zijlstra 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 Link: http://lkml.kernel.org/r/1402511843-4721-3-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar --- kernel/locking/mutex.c | 18 ++++++++---------- 1 file 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 #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;