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/[email protected]/
[2] https://lore.kernel.org/lkml/[email protected]/

Fixes: 5cdf2e3fea5e ("blkcg: associate blkg when associating a device")
Reported-and-tested-by: Ming Lei <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Signed-off-by: Dennis Zhou <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions