block: use trylock to avoid lockdep circular dependency in sysfs

Use trylock instead of blocking lock acquisition for update_nr_hwq_lock
in queue_requests_store() and elv_iosched_store() to avoid circular lock
dependency with kernfs active reference during concurrent disk deletion:

  update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
  kn->active -> update_nr_hwq_lock (via sysfs write path)

Return -EBUSY when the lock is not immediately available.

Reported-and-tested-by: Yi Zhang <[email protected]>
Closes: https://lore.kernel.org/linux-block/CAHj4cs-em-4acsHabMdT=jJhXkCzjnprD-aQH1OgrZo4nTnmMw@mail.gmail.com/
Fixes: 626ff4f ("blk-mq: convert to serialize updating nr_requests with update_nr_hwq_lock")
Signed-off-by: Ming Lei <[email protected]>
Tested-by: Yi Zhang <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

block/blk-sysfs.c

@@ -78,8 +78,14 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
 	/*
 	 * Serialize updating nr_requests with concurrent queue_requests_store()
 	 * and switching elevator.
+	 *
+	 * Use trylock to avoid circular lock dependency with kernfs active
+	 * reference during concurrent disk deletion:
+	 *   update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
+	 *   kn->active -> update_nr_hwq_lock (via this sysfs write path)
 	 */
-	down_write(&set->update_nr_hwq_lock);
+	if (!down_write_trylock(&set->update_nr_hwq_lock))
+		return -EBUSY;
 
 	if (nr == q->nr_requests)
 		goto unlock;

block/elevator.c

@@ -807,7 +807,16 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 	elv_iosched_load_module(ctx.name);
 	ctx.type = elevator_find_get(ctx.name);
 
-	down_read(&set->update_nr_hwq_lock);
+	/*
+	 * Use trylock to avoid circular lock dependency with kernfs active
+	 * reference during concurrent disk deletion:
+	 *   update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
+	 *   kn->active -> update_nr_hwq_lock (via this sysfs write path)
+	 */
+	if (!down_read_trylock(&set->update_nr_hwq_lock)) {
+		ret = -EBUSY;
+		goto out;
+	}
 	if (!blk_queue_no_elv_switch(q)) {
 		ret = elevator_change(q, &ctx);
 		if (!ret)
@@ -817,6 +826,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
 	}
 	up_read(&set->update_nr_hwq_lock);
 
+out:
 	if (ctx.type)
 		elevator_put(ctx.type);
 	return ret;