block: Change a rcu_read_{lock,unlock}_sched() pair into rcu_read_{lo…

…ck,unlock}()

scsi_device_quiesce() uses synchronize_rcu() to guarantee that the
effect of blk_set_preempt_only() will be visible for percpu_ref_tryget()
calls that occur after the queue unfreeze by using the approach
explained in https://lwn.net/Articles/573497/. The rcu read lock and
unlock calls in blk_queue_enter() form a pair with the synchronize_rcu()
call in scsi_device_quiesce(). Both scsi_device_quiesce() and
blk_queue_enter() must either use regular RCU or RCU-sched.
Since neither the RCU-protected code in blk_queue_enter() nor
blk_queue_usage_counter_release() sleeps, regular RCU protection
is sufficient. Note: scsi_device_quiesce() does not have to be
modified since it already uses synchronize_rcu().

Reported-by: Tejun Heo <[email protected]>
Fixes: 3a0a529 ("block, scsi: Make SCSI quiesce and resume work reliably")
Signed-off-by: Bart Van Assche <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Oleksandr Natalenko <[email protected]>
Cc: Martin Steigerwald <[email protected]>
Cc: [email protected] # v4.15
Signed-off-by: Jens Axboe <[email protected]>

block/blk-core.c

@@ -917,7 +917,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 		bool success = false;
 		int ret;
 
-		rcu_read_lock_sched();
+		rcu_read_lock();
 		if (percpu_ref_tryget_live(&q->q_usage_counter)) {
 			/*
 			 * The code that sets the PREEMPT_ONLY flag is
@@ -930,7 +930,7 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 				percpu_ref_put(&q->q_usage_counter);
 			}
 		}
-		rcu_read_unlock_sched();
+		rcu_read_unlock();
 
 		if (success)
 			return 0;