Commit f9e21aa9 authored by Waiman Long's avatar Waiman Long Committed by Peter Zijlstra
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locking/rwsem: No need to check for handoff bit if wait queue empty 



Since commit d257cc8c ("locking/rwsem: Make handoff bit handling
more consistent"), the handoff bit is always cleared if the wait queue
becomes empty. There is no need to check for RWSEM_FLAG_HANDOFF when
the wait list is known to be empty.

Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220322152059.2182333-2-longman@redhat.com
parent 8b023acc

kernel/locking/rwsem.c

+4 −5
Original line number Diff line number Diff line
@@ -977,12 +977,11 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat 
	if (list_empty(&sem->wait_list)) {

		/*

		 * In case the wait queue is empty and the lock isn't owned

		 * by a writer or has the handoff bit set, this reader can

		 * exit the slowpath and return immediately as its

		 * RWSEM_READER_BIAS has already been set in the count.

		 * by a writer, this reader can exit the slowpath and return

		 * immediately as its RWSEM_READER_BIAS has already been set

		 * in the count.

		 */

		if (!(atomic_long_read(&sem->count) &

		     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {

		if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {

			/* Provide lock ACQUIRE */

			smp_acquire__after_ctrl_dep();

			raw_spin_unlock_irq(&sem->wait_lock);