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 <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250808-ublk_quiesce2-v1-1-f87ade33fa3d@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 6561d2a561fa7ea570475494f0ed68d9ca85989a..6b95cf48ae77fff5661f9a7a2c7efdbcbcff7493 100644 (file)
--- a/drivers/block/ublk_drv.c
+++ b/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))
@@ -1644,7 +1645,6 @@ 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
@@ -1652,8 +1652,7 @@ static int ublk_ch_release(struct inode *inode, struct file *filp)
                 * 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);