ublk: don't quiesce in ublk_ch_release

ublk_ch_release currently quiesces the device's request_queue while
setting force_abort/fail_io.  This avoids data races by preventing
concurrent reads from the I/O path, but is not strictly needed - at this
point, canceling is already set and guaranteed to be observed by any
concurrently executing I/Os, so they will be handled properly even if
the changes to force_abort/fail_io propagate to the I/O path later.
Remove the quiesce/unquiesce calls from ublk_ch_release. This makes the
writes to force_abort/fail_io concurrent with the reads in the I/O path,
so make the accesses atomic.

Before this change, the call to blk_mq_quiesce_queue was responsible for
most (90%) of the runtime of ublk_ch_release. With that call eliminated,
ublk_ch_release runs much faster. Here is a comparison of the total time
spent in calls to ublk_ch_release when a server handling 128 devices
exits, before and after this change:

before: 1.11s
after: 0.09s

Signed-off-by: Uday Shankar <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: ps-ushankar

drivers/block/ublk_drv.c

@@ -1389,7 +1389,7 @@ static blk_status_t ublk_prep_req(struct ublk_queue *ubq, struct request *rq,
 {
 	blk_status_t res;
 
-	if (unlikely(ubq->fail_io))
+	if (unlikely(READ_ONCE(ubq->fail_io)))
 		return BLK_STS_TARGET;
 
 	/* With recovery feature enabled, force_abort is set in
@@ -1401,7 +1401,8 @@ static blk_status_t ublk_prep_req(struct ublk_queue *ubq, struct request *rq,
 	 * Note: force_abort is guaranteed to be seen because it is set
 	 * before request queue is unqiuesced.
 	 */
-	if (ublk_nosrv_should_queue_io(ubq) && unlikely(ubq->force_abort))
+	if (ublk_nosrv_should_queue_io(ubq) &&
+	    unlikely(READ_ONCE(ubq->force_abort)))
 		return BLK_STS_IOERR;
 
 	if (check_cancel && unlikely(ubq->canceling))
@@ -1644,16 +1645,14 @@ static int ublk_ch_release(struct inode *inode, struct file *filp)
 	 * Transition the device to the nosrv state. What exactly this
 	 * means depends on the recovery flags
 	 */
-	blk_mq_quiesce_queue(disk->queue);
 	if (ublk_nosrv_should_stop_dev(ub)) {
 		/*
 		 * Allow any pending/future I/O to pass through quickly
 		 * with an error. This is needed because del_gendisk
 		 * waits for all pending I/O to complete
 		 */
 		for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
-			ublk_get_queue(ub, i)->force_abort = true;
-		blk_mq_unquiesce_queue(disk->queue);
+			WRITE_ONCE(ublk_get_queue(ub, i)->force_abort, true);
 
 		ublk_stop_dev_unlocked(ub);
 	} else {
@@ -1663,9 +1662,8 @@ static int ublk_ch_release(struct inode *inode, struct file *filp)
 		} else {
 			ub->dev_info.state = UBLK_S_DEV_FAIL_IO;
 			for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
-				ublk_get_queue(ub, i)->fail_io = true;
+				WRITE_ONCE(ublk_get_queue(ub, i)->fail_io, true);
 		}
-		blk_mq_unquiesce_queue(disk->queue);
 	}
 unlock:
 	mutex_unlock(&ub->mutex);