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    Subject[PATCH] blkcg: handle dying request_queue when associating a blkg
    Date
    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: 5cdf2e3fea5e ("blkcg: associate blkg when associating a device")
    Reported-and-tested-by: Ming Lei <ming.lei@redhat.com>
    Signed-off-by: Dennis Zhou <dennis@kernel.org>
    ---
    block/blk-cgroup.c | 6 ++++++
    include/linux/blk-cgroup.h | 2 +-
    2 files changed, 7 insertions(+), 1 deletion(-)

    diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
    index 6bd0619a7d6e..c30661ddc873 100644
    --- a/block/blk-cgroup.c
    +++ b/block/blk-cgroup.c
    @@ -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;
    diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
    index bf13ecb0fe4f..f025fd1e22e6 100644
    --- a/include/linux/blk-cgroup.h
    +++ b/include/linux/blk-cgroup.h
    @@ -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;
    --
    2.17.1
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