From 33ecd2083a9560fbc1ef1b1279ef3ecb4c012a4f Mon Sep 17 00:00:00 2001 From: Jason Low Date: Mon, 14 Jul 2014 10:27:51 -0700 Subject: [PATCH] locking/spinlocks/mcs: Micro-optimize osq_unlock() In the unlock function of the cancellable MCS spinlock, the first thing we do is to retrive the current CPU's osq node. However, due to the changes made in the previous patch, in the common case where the lock is not contended, we wouldn't need to access the current CPU's osq node anymore. This patch optimizes this by only retriving this CPU's osq node after we attempt the initial cmpxchg to unlock the osq and found that its contended. Signed-off-by: Jason Low Signed-off-by: Peter Zijlstra Cc: Scott Norton Cc: "Paul E. McKenney" Cc: Dave Chinner Cc: Waiman Long Cc: Davidlohr Bueso Cc: Rik van Riel Cc: Andrew Morton Cc: "H. Peter Anvin" Cc: Steven Rostedt Cc: Tim Chen Cc: Konrad Rzeszutek Wilk Cc: Aswin Chandramouleeswaran Cc: Linus Torvalds Link: http://lkml.kernel.org/r/1405358872-3732-5-git-send-email-jason.low2@hp.com Signed-off-by: Ingo Molnar --- kernel/locking/mcs_spinlock.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c index 32fc16c0a545..be9ee1559fca 100644 --- a/kernel/locking/mcs_spinlock.c +++ b/kernel/locking/mcs_spinlock.c @@ -182,8 +182,7 @@ unqueue: void osq_unlock(struct optimistic_spin_queue *lock) { - struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); - struct optimistic_spin_node *next; + struct optimistic_spin_node *node, *next; int curr = encode_cpu(smp_processor_id()); /* @@ -195,6 +194,7 @@ void osq_unlock(struct optimistic_spin_queue *lock) /* * Second most likely case. */ + node = this_cpu_ptr(&osq_node); next = xchg(&node->next, NULL); if (next) { ACCESS_ONCE(next->locked) = 1;