[PATCH v4 2/2] locking/rwsem: check for active writer before wakeup

[Waiman Long <Waiman.Long@hp.com>]

On a highly contended rwsem, spinlock contention due to the slow
rwsem_wake() call can be a significant portion of the total CPU cycles
used. With writer lock stealing and writer optimistic spinning, there
is also a chance that the lock may have been stolen by the time that
the wait_lock is acquired.

This patch adds a low cost checking code after acquiring the wait_lock
to look for active writer. The presence of an active writer will
abort the wakeup operation.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
 kernel/locking/rwsem-xadd.c |   21 +++++++++++++++++++--
 1 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2bb25e2..815f0cc 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -399,6 +399,15 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
 	return osq_is_locked(&sem->osq);
 }
 
+/*
+ * Return true if there is an active writer by checking the owner field which
+ * should be set if there is one.
+ */
+static inline bool rwsem_has_active_writer(struct rw_semaphore *sem)
+{
+	return READ_ONCE(sem->owner) != NULL;
+}
+
 #else
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 {
@@ -409,6 +418,11 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
 {
 	return false;
 }
+
+static inline bool rwsem_has_active_writer(struct rw_semaphore *sem)
+{
+	return false;	/* Assume it has no active writer */
+}
 #endif
 
 /*
@@ -524,8 +538,11 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
 	raw_spin_lock_irqsave(&sem->wait_lock, flags);
 locked:
 
-	/* do nothing if list empty */
-	if (!list_empty(&sem->wait_list))
+	/*
+	 * Do nothing if list empty or the lock has just been stolen by a
+	 * writer after a possibly long wait in getting the wait_lock.
+	 */
+	if (!list_empty(&sem->wait_list) && !rwsem_has_active_writer(sem))
 		sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
 
 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
-- 
1.7.1