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 <long...@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
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