From 72d5305dcb3637913c2c37e847a4de9028e49244 Mon Sep 17 00:00:00 2001 From: Jason Low <jason.low2@hp.com> Date: Wed, 11 Jun 2014 11:37:23 -0700 Subject: [PATCH] locking/mutexes: Optimize mutex trylock slowpath The mutex_trylock() function calls into __mutex_trylock_fastpath() when trying to obtain the mutex. On 32 bit x86, in the !__HAVE_ARCH_CMPXCHG case, __mutex_trylock_fastpath() calls directly into __mutex_trylock_slowpath() regardless of whether or not the mutex is locked. In __mutex_trylock_slowpath(), we then acquire the wait_lock spinlock, xchg() lock->count with -1, then set lock->count back to 0 if there are no waiters, and return true if the prev lock count was 1. However, if the mutex is already locked, then there isn't much point in attempting all of the above expensive operations. In this patch, we only attempt the above trylock operations if the mutex is unlocked. Signed-off-by: Jason Low <jason.low2@hp.com> Reviewed-by: Davidlohr Bueso <davidlohr@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: Waiman.Long@hp.com Cc: scott.norton@hp.com Cc: aswin@hp.com Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/1402511843-4721-5-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org> --- kernel/locking/mutex.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index e4d997bb7d70a..11b103d87b278 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -820,6 +820,10 @@ static inline int __mutex_trylock_slowpath(atomic_t *lock_count) unsigned long flags; int prev; + /* No need to trylock if the mutex is locked. */ + if (mutex_is_locked(lock)) + return 0; + spin_lock_mutex(&lock->wait_lock, flags); prev = atomic_xchg(&lock->count, -1); -- 2.30.2