Commit aa68744f authored Nov 09, 2015 by Waiman Long Committed by Ingo Molnar Nov 23, 2015

locking/qspinlock: Avoid redundant read of next pointer



With optimistic prefetch of the next node cacheline, the next pointer
may have been properly inititalized. As a result, the reading
of node->next in the contended path may be redundant. This patch
eliminates the redundant read if the next pointer value is not NULL.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447114167-47185-4-git-send-email-Waiman.Long@hpe.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 81b55986

kernel/locking/qspinlock.c

+6 −3

Original line number	Diff line number	Diff line
		@@ -396,6 +396,7 @@ queue:
		* p,, -> n,,
		*/
		old = xchg_tail(lock, tail);
		next = NULL;

		/*
		* if there was a previous node; link it and wait until reaching the
		@@ -463,10 +464,12 @@ queue:
		}

		/*
		* contended path; wait for next, release.
		* contended path; wait for next if not observed yet, release.
		*/
		if (!next) {
		while (!(next = READ_ONCE(node->next)))
		cpu_relax();
		}

		arch_mcs_spin_unlock_contended(&next->locked);
		pv_kick_node(lock, next);

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