Commit b247be3f authored by Will Deacon's avatar Will Deacon Committed by Ingo Molnar
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locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound

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: default avatarWaiman Long <>
Signed-off-by: default avatarWill Deacon <>
Acked-by: default avatarPeter Zijlstra (Intel) <>
Acked-by: default avatarWaiman Long <>
Cc: Linus Torvalds <>
Cc: Thomas Gleixner <>

Signed-off-by: default avatarIngo Molnar <>
parent 6512276d
......@@ -7,6 +7,8 @@
#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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