[PATCH v2 08/13] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node

[Will Deacon]

When a locker reaches the head of the queue and takes the lock, a
concurrent locker may enqueue and force the lock holder to spin
whilst its node->next field is initialised. Rather than open-code
a READ_ONCE/cpu_relax() loop, this can be implemented using
smp_cond_load_relaxed instead.

Cc: Peter Zijlstra 
Cc: Ingo Molnar 
Signed-off-by: Will Deacon 
---
 kernel/locking/qspinlock.c | 6 ++
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 648a16a2cd23..c781ddbe59a6 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -523,10 +523,8 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
/*
 * contended path; wait for next if not observed yet, release.
 */
-   if (!next) {
-   while (!(next = READ_ONCE(node->next)))
-   cpu_relax();
-   }
+   if (!next)
+   next = smp_cond_load_relaxed(>next, (VAL));
 
arch_mcs_spin_unlock_contended(>locked);
pv_kick_node(lock, next);
-- 
2.1.4

[PATCH v2 08/13] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node

[Will Deacon]

When a locker reaches the head of the queue and takes the lock, a
concurrent locker may enqueue and force the lock holder to spin
whilst its node->next field is initialised. Rather than open-code
a READ_ONCE/cpu_relax() loop, this can be implemented using
smp_cond_load_relaxed instead.

Cc: Peter Zijlstra 
Cc: Ingo Molnar 
Signed-off-by: Will Deacon 
---
 kernel/locking/qspinlock.c | 6 ++
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 648a16a2cd23..c781ddbe59a6 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -523,10 +523,8 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
/*
 * contended path; wait for next if not observed yet, release.
 */
-   if (!next) {
-   while (!(next = READ_ONCE(node->next)))
-   cpu_relax();
-   }
+   if (!next)
+   next = smp_cond_load_relaxed(>next, (VAL));
 
arch_mcs_spin_unlock_contended(>locked);
pv_kick_node(lock, next);
-- 
2.1.4