[PATCH] blk-throttle: fix null pointer dereference while throttling writeback IOs

[xuejiufei]

From: Jiufei Xue <jiufei....@alibaba-inc.com>

A null pointer dereference can occur when blkcg is removed manually
with writeback IOs inflight. This is caused by the following case:

Writeback kworker submit the bio and set bio->bi_cg_private to tg
in blk_throtl_assoc_bio.
Then we remove the block cgroup manually, the blkg and tg would be
freed if there is no request inflight.
When the submitted bio come back, blk_throtl_bio_endio() fetch the tg
which was already freed.

Fix this by increasing the refcount of blkg in funcion
blk_throtl_assoc_bio() so that the blkg will not be freed until the
bio_endio called.

Signed-off-by: Jiufei Xue <jiufei....@alibaba-inc.com>
---
 block/blk-throttle.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 17816a0..d80c3f0 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2112,8 +2112,12 @@ static inline void throtl_update_latency_buckets(struct 
throtl_data *td)
 static void blk_throtl_assoc_bio(struct throtl_grp *tg, struct bio *bio)
 {
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
-       if (bio->bi_css)
+       if (bio->bi_css) {
+               if (bio->bi_cg_private)
+                       blkg_put(tg_to_blkg(bio->bi_cg_private));
                bio->bi_cg_private = tg;
+               blkg_get(tg_to_blkg(tg));
+       }
        blk_stat_set_issue(&bio->bi_issue_stat, bio_sectors(bio));
 #endif
 }
@@ -2283,8 +2287,10 @@ void blk_throtl_bio_endio(struct bio *bio)
 
        start_time = blk_stat_time(&bio->bi_issue_stat) >> 10;
        finish_time = __blk_stat_time(finish_time_ns) >> 10;
-       if (!start_time || finish_time <= start_time)
+       if (!start_time || finish_time <= start_time) {
+               blkg_put(tg_to_blkg(tg));
                return;
+       }
 
        lat = finish_time - start_time;
        /* this is only for bio based driver */
@@ -2314,6 +2320,8 @@ void blk_throtl_bio_endio(struct bio *bio)
                tg->bio_cnt /= 2;
                tg->bad_bio_cnt /= 2;
        }
+
+       blkg_put(tg_to_blkg(tg));
 }
 #endif
 
-- 
1.9.4