From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932121AbaFKShh (ORCPT ); Wed, 11 Jun 2014 14:37:37 -0400 Received: from g4t3426.houston.hp.com ([15.201.208.54]:18798 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755190AbaFKShf (ORCPT ); Wed, 11 Jun 2014 14:37:35 -0400 From: Jason Low To: mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, tim.c.chen@linux.intel.com, paulmck@linux.vnet.ibm.com, rostedt@goodmis.org, davidlohr@hp.com, Waiman.Long@hp.com, scott.norton@hp.com, aswin@hp.com, jason.low2@hp.com Subject: [PATCH v2 2/4] mutex: Delete the MUTEX_SHOW_NO_WAITER macro Date: Wed, 11 Jun 2014 11:37:21 -0700 Message-Id: <1402511843-4721-3-git-send-email-jason.low2@hp.com> X-Mailer: git-send-email 1.7.9.5 In-Reply-To: <1402511843-4721-1-git-send-email-jason.low2@hp.com> References: <1402511843-4721-1-git-send-email-jason.low2@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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 #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; -- 1.7.1