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blkcg: handle dying request_queue when associating a blkg

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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: 5cdf2e3fea ("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>
This commit is contained in:
Dennis Zhou 2018-12-11 18:03:08 -05:00 committed by Jens Axboe
parent 544fbd16a4
commit 0273ac349f
2 changed files with 7 additions and 1 deletions
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6
block/blk-cgroup.c
View File

@ -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;

2
include/linux/blk-cgroup.h
View File

@ -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;