On Fri, Jan 12, 2018 at 10:06:04AM -0500, Mike Snitzer wrote:
> The original commit e9a823fb34a8b (block: fix warning when I/O elevator
> is changed as request_queue is being removed) is pretty conflated.
> "conflated" because the resource being protected by q->sysfs_lock isn't
> the queue_flags (it is the 'queue' kobj).
>
> q->sysfs_lock serializes __elevator_change() (via elv_iosched_store)
> from racing with blk_unregister_queue():
> 1) By holding q->sysfs_lock first, __elevator_change() can complete
> before a racing blk_unregister_queue().
> 2) Conversely, __elevator_change() is testing for QUEUE_FLAG_REGISTERED
> in case elv_iosched_store() loses the race with blk_unregister_queue(),
> it needs a way to know the 'queue' kobj isn't there.
>
> Expand the scope of blk_unregister_queue()'s q->sysfs_lock use so it is
> held until after the 'queue' kobj is removed.
This way will cause deadlock, see blow.
>
> Also, blk_unregister_queue() should use q->queue_lock to protect against
> any concurrent writes to q->queue_flags -- even though chances are the
> queue is being cleaned up so no concurrent writes are likely.
>
> Fixes: e9a823fb34a8b ("block: fix warning when I/O elevator is changed as
> request_queue is being removed")
> Signed-off-by: Mike Snitzer <snit...@redhat.com>
> ---
> block/blk-sysfs.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 870484eaed1f..9272452ff456 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -929,12 +929,17 @@ void blk_unregister_queue(struct gendisk *disk)
> if (WARN_ON(!q))
> return;
>
> + /*
> + * Protect against the 'queue' kobj being accessed
> + * while/after it is removed.
> + */
> mutex_lock(&q->sysfs_lock);
> - queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
> - mutex_unlock(&q->sysfs_lock);
>
> - wbt_exit(q);
> + spin_lock_irq(q->queue_lock);
> + queue_flag_clear(QUEUE_FLAG_REGISTERED, q);
> + spin_unlock_irq(q->queue_lock);
>
> + wbt_exit(q);
>
> if (q->mq_ops)
> blk_mq_unregister_dev(disk_to_dev(disk), q);
void blk_mq_unregister_dev(struct device *dev, struct request_queue *q)
{
mutex_lock(&q->sysfs_lock);
__blk_mq_unregister_dev(dev, q);
mutex_unlock(&q->sysfs_lock);
}
> @@ -946,4 +951,6 @@ void blk_unregister_queue(struct gendisk *disk)
> kobject_del(&q->kobj);
> blk_trace_remove_sysfs(disk_to_dev(disk));
> kobject_put(&disk_to_dev(disk)->kobj);
> +
> + mutex_unlock(&q->sysfs_lock);
> }
Except for above, I remember there is also lockdep warning between
sysfs_lock and driver core's lock if the sysfs_lock is extended in this
way(I tried it before, but forget the details now), so please just hold
queue_lock inside the sysfs lock.
Thanks,
Ming
