Commit a063057d authored by Bart Van Assche's avatar Bart Van Assche Committed by Jens Axboe

block: Fix a race between request queue removal and the block cgroup controller

Avoid that the following race can occur:

blk_cleanup_queue()               blkcg_print_blkgs()
  spin_lock_irq(lock) (1)           spin_lock_irq(blkg->q->queue_lock) (2,5)
    q->queue_lock = &q->__queue_lock (3)
  spin_unlock_irq(lock) (4)
                                    spin_unlock_irq(blkg->q->queue_lock) (6)

(1) take driver lock;
(2) busy loop for driver lock;
(3) override driver lock with internal lock;
(4) unlock driver lock;
(5) can take driver lock now;
(6) but unlock internal lock.

This change is safe because only the SCSI core and the NVME core keep
a reference on a request queue after having called blk_cleanup_queue().
Neither driver accesses any of the removed data structures between its
blk_cleanup_queue() and blk_put_queue() calls.
Reported-by: default avatarJoseph Qi <[email protected]>
Signed-off-by: default avatarBart Van Assche <[email protected]>
Reviewed-by: default avatarJoseph Qi <[email protected]>
Cc: Jan Kara <[email protected]>
Signed-off-by: default avatarJens Axboe <[email protected]>
parent 498f6650
......@@ -719,6 +719,37 @@ void blk_cleanup_queue(struct request_queue *q)
del_timer_sync(&q->backing_dev_info->laptop_mode_wb_timer);
blk_sync_queue(q);
/*
* I/O scheduler exit is only safe after the sysfs scheduler attribute
* has been removed.
*/
WARN_ON_ONCE(q->kobj.state_in_sysfs);
/*
* Since the I/O scheduler exit code may access cgroup information,
* perform I/O scheduler exit before disassociating from the block
* cgroup controller.
*/
if (q->elevator) {
ioc_clear_queue(q);
elevator_exit(q, q->elevator);
q->elevator = NULL;
}
/*
* Remove all references to @q from the block cgroup controller before
* restoring @q->queue_lock to avoid that restoring this pointer causes
* e.g. blkcg_print_blkgs() to crash.
*/
blkcg_exit_queue(q);
/*
* Since the cgroup code may dereference the @q->backing_dev_info
* pointer, only decrease its reference count after having removed the
* association with the block cgroup controller.
*/
bdi_put(q->backing_dev_info);
if (q->mq_ops)
blk_mq_free_queue(q);
percpu_ref_exit(&q->q_usage_counter);
......
......@@ -798,13 +798,6 @@ static void __blk_release_queue(struct work_struct *work)
if (test_bit(QUEUE_FLAG_POLL_STATS, &q->queue_flags))
blk_stat_remove_callback(q, q->poll_cb);
blk_stat_free_callback(q->poll_cb);
bdi_put(q->backing_dev_info);
blkcg_exit_queue(q);
if (q->elevator) {
ioc_clear_queue(q);
elevator_exit(q, q->elevator);
}
blk_free_queue_stats(q->stats);
......
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