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locking/mcs: Use smp_cond_load_acquire() in MCS spin loop

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For qspinlocks on ARM64, we would like to use WFE instead
of purely spinning. Qspinlocks internally have lock
contenders spin on an MCS lock.

Update arch_mcs_spin_lock_contended() such that it uses
the new smp_cond_load_acquire() so that ARM64 can also
override this spin loop with its own implementation using WFE.

On x86, this can also be cheaper than spinning on
smp_load_acquire().

Signed-off-by: Jason Low <jason.low2@hp.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1524738868-31318-9-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Jason Low 2018-04-26 11:34:22 +01:00 committed by Ingo Molnar
parent f9c811fac4
commit 7f56b58a92
1 changed files with 6 additions and 4 deletions
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10
kernel/locking/mcs_spinlock.h
View file

@ -23,13 +23,15 @@ struct mcs_spinlock {
#ifndef arch_mcs_spin_lock_contended
/*
* Using smp_load_acquire() provides a memory barrier that ensures
* subsequent operations happen after the lock is acquired.
* Using smp_cond_load_acquire() provides the acquire semantics
* required so that subsequent operations happen after the
* lock is acquired. Additionally, some architectures such as
* ARM64 would like to do spin-waiting instead of purely
* spinning, and smp_cond_load_acquire() provides that behavior.
*/
#define arch_mcs_spin_lock_contended(l) \
do { \
while (!(smp_load_acquire(l))) \
cpu_relax(); \
smp_cond_load_acquire(l, VAL); \
} while (0)
#endif