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().

---
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(-)

index c835270..c19ce41 100644
@@ -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