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From:   Dennis Zhou <dennis@kernel.org>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     kernel-team@fb.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Dennis Zhou <dennis@kernel.org>
Subject: [PATCH] blkcg: add rcu lock to bio_clone_blkg_association()
Date:   Fri, 21 Dec 2018 08:54:41 -0600
Message-Id: <20181221145441.13814-1-dennis@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

I cleaned up blkg_tryget_closest() to require rcu_read_lock() earlier.
However, this was a subtle case too which clearly was too subtle for me.
The idea was the src bio should be holding a ref to the blkg so rcu
wasn't technically needed. If it doesn't hold a ref, it should be %NULL
and the blkg->parent pointers are unused.

This adds the appropriate read lock in bio_clone_blkg_association().

Fixes: 80fd3c272c1a ("blkcg: clean up blkg_tryget_closest()")
Reported-by: syzbot+a36a3ba92bea3b315c64@syzkaller.appspotmail.com
Signed-off-by: Dennis Zhou <dennis@kernel.org>
---
 block/bio.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/bio.c b/block/bio.c
index c288b9057042..9194d8ad3d5e 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2096,8 +2096,12 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
  */
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
+	rcu_read_lock();
+
 	if (src->bi_blkg)
 		__bio_associate_blkg(dst, src->bi_blkg);
+
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
 #endif /* CONFIG_BLK_CGROUP */
-- 
2.17.1