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| author | Waiman Long <Waiman.Long@hpe.com> | 2015-11-09 19:09:23 -0500 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2015-11-23 10:02:00 +0100 |
| commit | aa68744f80bfb6f26fbe7f10e42876066f7dac1b (patch) | |
| tree | 1346f724cd714045744db3c49c6b859b10c3b1a3 /kernel/locking | |
| parent | 81b5598665a24083dd889fbd8cb08b0d8de4b8ad (diff) | |
| download | op-kernel-dev-aa68744f80bfb6f26fbe7f10e42876066f7dac1b.zip op-kernel-dev-aa68744f80bfb6f26fbe7f10e42876066f7dac1b.tar.gz | |
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>
Diffstat (limited to 'kernel/locking')
| -rw-r--r-- | kernel/locking/qspinlock.c | 9 |
1 files 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); |
