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locking/qspinlock/x86: Fix performance regression under unaccelerated VMs

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Dave ran into horrible performance on a VM without PARAVIRT_SPINLOCKS
set and Linus noted that the test-and-set implementation was retarded.

One should spin on the variable with a load, not a RMW.

While there, remove 'queued' from the name, as the lock isn't queued
at all, but a simple test-and-set.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Dave Chinner <david@fromorbit.com>
Tested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: stable@vger.kernel.org # v4.2+
Link: http://lkml.kernel.org/r/20150904152523.GR18673@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2015-09-04 17:25:23 +02:00
committed by Ingo Molnar
parent edcd591c77
commit 43b3f02899
3 changed files with 15 additions and 7 deletions
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4
include/asm-generic/qspinlock.h
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@@ -111,8 +111,8 @@ static inline void queued_spin_unlock_wait(struct qspinlock *lock)
cpu_relax();
}
#ifndef virt_queued_spin_lock
static __always_inline bool virt_queued_spin_lock(struct qspinlock *lock)
#ifndef virt_spin_lock
static __always_inline bool virt_spin_lock(struct qspinlock *lock)
{
return false;
}