Commit 0273ac34 authored Dec 11, 2018 by Dennis Zhou Committed by Jens Axboe Dec 12, 2018

blkcg: handle dying request_queue when associating a blkg

Between v3 [1] and v4 [2] of the blkg association series, the
association point moved from generic_make_request_checks(), which is
called after the request enters the queue, to bio_set_dev(), which is when
the bio is formed before submit_bio(). When the request_queue goes away,
the blkgs supporting the request_queue are destroyed and then the
q->root_blkg is set to %NULL.

This patch adds a %NULL check to blkg_tryget_closest() to prevent the
NPE caused by the above. It also adds a guard to see if the
request_queue is dying when creating a blkg to prevent creating a blkg
for a dead request_queue.

[1] https://lore.kernel.org/lkml/20180911184137.35897-1-dennisszhou@gmail.com/
[2] https://lore.kernel.org/lkml/20181126211946.77067-1-dennis@kernel.org/



Fixes: 5cdf2e3f ("blkcg: associate blkg when associating a device")
Reported-and-tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 544fbd16

block/blk-cgroup.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -202,6 +202,12 @@ static struct blkcg_gq blkg_create(struct blkcg blkcg,
		WARN_ON_ONCE(!rcu_read_lock_held());
		lockdep_assert_held(&q->queue_lock);

		/* request_queue is dying, do not create/recreate a blkg */
		if (blk_queue_dying(q)) {
		ret = -ENODEV;
		goto err_free_blkg;
		}

		/* blkg holds a reference to blkcg */
		if (!css_tryget_online(&blkcg->css)) {
		ret = -ENODEV;

include/linux/blk-cgroup.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -511,7 +511,7 @@ static inline bool blkg_tryget(struct blkcg_gq *blkg)
		*/
		static inline struct blkcg_gq blkg_tryget_closest(struct blkcg_gq blkg)
		{
		while (!percpu_ref_tryget(&blkg->refcnt))
		while (blkg && !percpu_ref_tryget(&blkg->refcnt))
		blkg = blkg->parent;

		return blkg;

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