[PATCH v5 9/9] locking/rwsem: Enable reader lock stealing

[Waiman Long <longman@redhat.com>]

The rwsem has supported writer lock stealing for a long time. Reader
lock stealing isn't allowed as it may lead to writer lock starvation.
As a result, writers are preferred over readers. However, preferring
readers generally leads to better overall performance.

This patch now enables reader lock stealing on a rwsem as long as
the lock is reader-owned and optimistic spinning hasn't been disabled
because of long writer wait. This will improve overall performance
without running the risk of writer lock starvation.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/rwsem-xadd.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index a571bec..f5caba8 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -529,6 +529,14 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 		goto enqueue;
 
 	/*
+	 * Steal the lock if no writer was present and the optimistic
+	 * spinning disable bit isn't set.
+	 */
+	count = atomic_long_read(&sem->count);
+	if (!count_has_writer(count))
+		return sem;
+
+	/*
 	 * Undo read bias from down_read operation to stop active locking if:
 	 * 1) Optimistic spinners are present;
 	 * 2) the wait_lock isn't free; or
-- 
1.8.3.1