2016-04-20 19:53:00

by Jason Low

[permalink] [raw]
Subject: [PATCH v3] MCS spinlock: Use smp_cond_load_acquire() in spin loop

This patch applies on top of:

git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/rfc

---
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 <[email protected]>
---
v2 -> v3:
- Add additional comments about the use of smp_cond_load_acquire().

v1 -> v2:
- Pass l instead of &l to smp_cond_load_acquire() since
l is already a pointer to the lock variable.
kernel/locking/mcs_spinlock.h | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index c835270..c19ce41 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -22,13 +22,15 @@ 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_lowlatency(); \
+ smp_cond_load_acquire(l, VAL); \
} while (0)
#endif

--
2.1.4