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@xxxxxxxxxx> --- 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
