blkcg: fix edge case for blk_get_rl() under memory pressure

It is possible for blkg creation to fail when in blk_get_rl(). In this
situation, the fallback logic returns the nearest created blkg. There is
however special handling for the request_list for the root blkcg. This
fixes the missing edge case from the earlier series changing
blk_get_rl().

Fixes: e2b0989954ae ("blkcg: cleanup and make blk_get_rl use blkg_lookup_create")
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index b7fd080..1e76cee 100644 (file)
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -597,7 +597,7 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
        if (unlikely(!blkg))
                blkg = __blkg_lookup_create(blkcg, q);
 
-       if (!blkg_tryget(blkg))
+       if (blkg->blkcg == &blkcg_root || !blkg_tryget(blkg))
                goto rl_use_root;
 
        rcu_read_unlock();