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 <tj@xxxxxxxxxx> Fixes: 3a0a529971ec ("block, scsi: Make SCSI quiesce and resume work reliably") Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxx> Acked-by: Tejun Heo <tj@xxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Hannes Reinecke <hare@xxxxxxxx> Cc: Ming Lei <ming.lei@xxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: Johannes Thumshirn <jthumshirn@xxxxxxx> Cc: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx> Cc: Martin Steigerwald <martin@xxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx # v4.15 --- Changes between v1 and v2: - Explained in the interaction between scsi_device_quiesce() and blk_queue_enter() in the patch description. block/blk-core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index 5e88c579e896..a0f675f84f86 100644 --- a/block/blk-core.c +++ b/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; -- 2.16.2
