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Subject[PATCH 4.14 28/72] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound
4.14-stable review patch.  If anyone has any objections, please let me know.


commit b247be3fe89b6aba928bf80f4453d1c4ba8d2063 upstream.

On x86, atomic_cond_read_relaxed will busy-wait with a cpu_relax() loop,
so it is desirable to increase the number of times we spin on the qspinlock
lockword when it is found to be transitioning from pending to locked.

According to Waiman Long:

| Ideally, the spinning times should be at least a few times the typical
| cacheline load time from memory which I think can be down to 100ns or
| so for each cacheline load with the newest systems or up to several
| hundreds ns for older systems.

which in his benchmarking corresponded to 512 iterations.

Suggested-by: Waiman Long <>
Signed-off-by: Will Deacon <>
Acked-by: Peter Zijlstra (Intel) <>
Acked-by: Waiman Long <>
Cc: Linus Torvalds <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>
Signed-off-by: Sebastian Andrzej Siewior <>
Signed-off-by: Sasha Levin <>
arch/x86/include/asm/qspinlock.h | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index cf4cdf508ef4..2cb6624acaec 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -6,6 +6,8 @@
#include <asm-generic/qspinlock_types.h>
#include <asm/paravirt.h>

+#define _Q_PENDING_LOOPS (1 << 9)
#define queued_spin_unlock queued_spin_unlock
* queued_spin_unlock - release a queued spinlock

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