From 42fa566bd74aa7b95413fb00611ec983b488222d Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Wed, 30 Jul 2014 13:41:51 -0700 Subject: [PATCH] locking/mutexes: Document quick lock release when unlocking When unlocking, we always want to reach the slowpath with the lock's counter indicating it is unlocked. -- as returned by the asm fastpath call or by explicitly setting it. While doing so, at least in theory, we can optimize and allow faster lock stealing. When unlocking, we always want to reach the slowpath with the lock's counter indicating it is unlocked. -- as returned by the asm fastpath call or by explicitly setting it. While doing so, at least in theory, we can optimize and allow faster lock stealing. Signed-off-by: Davidlohr Bueso Signed-off-by: Peter Zijlstra Cc: jason.low2@hp.com Cc: aswin@hp.com Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1406752916-3341-2-git-send-email-davidlohr@hp.com Signed-off-by: Ingo Molnar --- kernel/locking/mutex.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index ad0e3335c481..93bec48f09ed 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -684,9 +684,16 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested) unsigned long flags; /* - * some architectures leave the lock unlocked in the fastpath failure + * As a performance measurement, release the lock before doing other + * wakeup related duties to follow. This allows other tasks to acquire + * the lock sooner, while still handling cleanups in past unlock calls. + * This can be done as we do not enforce strict equivalence between the + * mutex counter and wait_list. + * + * + * Some architectures leave the lock unlocked in the fastpath failure * case, others need to leave it locked. In the later case we have to - * unlock it here + * unlock it here - as the lock counter is currently 0 or negative. */ if (__mutex_slowpath_needs_to_unlock()) atomic_set(&lock->count, 1); -- 2.39.2