locking/rwsem: No need to check for handoff bit if wait queue empty

Since commit d257cc8 ("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: Waiman Long <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: Waiman-Long

kernel/locking/rwsem.c

@@ -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);