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 <jason.low2@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Scott Norton <scott.norton@hp.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Waiman Long <waiman.long@hp.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1405358872-3732-5-git-send-email-jason.low2@hp.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/mcs_spinlock.c

@@ -182,8 +182,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 
 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;