[PATCH] cfq: clear queue pointers from cfqg after unpinning them in cfq_pd_offline

The current linux-block, 4.18 and 4.17 can reliably be crashed within few
minutes by running the following bash snippet:

mkfs.ext4 -v /dev/sda3 && mount /dev/sda3 /mnt/test/ -t ext4;
while true; do
	mkdir /sys/fs/cgroup/unified/test/;
	echo $$ >/sys/fs/cgroup/unified/test/cgroup.procs;
	dd if=/dev/zero of=/mnt/test/test-$(( RANDOM * 10 / 32768 )) bs=1M count=1024 &
	echo $$ >/sys/fs/cgroup/unified/cgroup.procs;
	sleep 1;
	kill -KILL $!; wait $!;
	rmdir /sys/fs/cgroup/unified/test;
done

# cat /sys/block/sda/queue/scheduler
noop [cfq]
# cat /sys/block/sda/queue/rotational
1
# cat /sys/fs/cgroup/unified/cgroup.subtree_control
cpu io memory pids

The backtraces vary but often they are NULL pointer dereferences due to
various cfqq fields being NULL.
Or BUG_ON(cfqq->ref <= 0) in cfq_put_queue() triggers due to cfqq reference
count being zero.

Bisection points at
commit 4c6994806f70 ("blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()").
The prime suspect looked like .pd_offline_fn() method being called multiple
times, but from analyzing the mentioned commit this didn't seem possible
and runtime trials have confirmed that.

However, CFQ's cfq_pd_offline() implementation of the above method were
leaving queue pointers intact in cfqg after unpinning them.
After making sure that they are cleared to NULL in this function I can no
longer reproduce the crash.

Signed-off-by: Maciej S. Szmigiero <redacted>
Fixes: 4c6994806f70 ("blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()").
Cc: stable@vger.kernel.org
---
 block/cfq-iosched.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 2eb87444b157..ed41aa978c4a 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c

@@ -1644,14 +1644,20 @@ static void cfq_pd_offline(struct blkg_policy_data *pd)
 	int i;
 
 	for (i = 0; i < IOPRIO_BE_NR; i++) {
-		if (cfqg->async_cfqq[0][i])
+		if (cfqg->async_cfqq[0][i]) {
 			cfq_put_queue(cfqg->async_cfqq[0][i]);
-		if (cfqg->async_cfqq[1][i])
+			cfqg->async_cfqq[0][i] = NULL;
+		}
+		if (cfqg->async_cfqq[1][i]) {
 			cfq_put_queue(cfqg->async_cfqq[1][i]);
+			cfqg->async_cfqq[1][i] = NULL;
+		}
 	}
 
-	if (cfqg->async_idle_cfqq)
+	if (cfqg->async_idle_cfqq) {
 		cfq_put_queue(cfqg->async_idle_cfqq);
+		cfqg->async_idle_cfqq = NULL;
+	}
 
 	/*
 	 * @blkg is going offline and will be ignored by