Commit-ID: aa68744f80bfb6f26fbe7f10e42876066f7dac1b Gitweb: http://git.kernel.org/tip/aa68744f80bfb6f26fbe7f10e42876066f7dac1b Author: Waiman Long <waiman.l...@hpe.com> AuthorDate: Mon, 9 Nov 2015 19:09:23 -0500 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 23 Nov 2015 10:02:00 +0100
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.l...@hpe.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Davidlohr Bueso <d...@stgolabs.net> Cc: Douglas Hatch <doug.ha...@hpe.com> Cc: H. Peter Anvin <h...@zytor.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Scott J Norton <scott.nor...@hpe.com> Cc: Thomas Gleixner <t...@linutronix.de> Link: http://lkml.kernel.org/r/1447114167-47185-4-git-send-email-waiman.l...@hpe.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/locking/qspinlock.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 365b203..9862078 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -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. */ - while (!(next = READ_ONCE(node->next))) - cpu_relax(); + if (!next) { + while (!(next = READ_ONCE(node->next))) + cpu_relax(); + } arch_mcs_spin_unlock_contended(&next->locked); pv_kick_node(lock, next); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
