Re: [PATCH] cfq: clear queue pointers from cfqg after unpinning them in cfq_pd_offline
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From: Paolo Valente <hidden>
Date: 2018-08-17 17:30:18
Also in:
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Il giorno 17 ago 2018, alle ore 19:28, Maciej S. Szmigiero =
[off-list ref] ha scritto:
=20 The current linux-block, 4.18 and 4.17 can reliably be crashed within =
few
minutes by running the following bash snippet: =20 mkfs.ext4 -v /dev/sda3 && mount /dev/sda3 /mnt/test/ -t ext4; while true; do mkdir /sys/fs/cgroup/unified/test/; echo $$ >/sys/fs/cgroup/unified/test/cgroup.procs; dd if=3D/dev/zero of=3D/mnt/test/test-$(( RANDOM * 10 / 32768 )) =
bs=3D1M count=3D1024 &
echo $$ >/sys/fs/cgroup/unified/cgroup.procs; sleep 1; kill -KILL $!; wait $!; rmdir /sys/fs/cgroup/unified/test; done =20 # 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 =20 The backtraces vary but often they are NULL pointer dereferences due =
to
various cfqq fields being NULL. Or BUG_ON(cfqq->ref <=3D 0) in cfq_put_queue() triggers due to cfqq =
reference
count being zero.
=20
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. =20 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. =20
By chance, did you check whether is BFQ is ok in this respect? Thanks, Paolo
Signed-off-by: Maciej S. Szmigiero <redacted>
Fixes: 4c6994806f70 ("blk-throttle: fix race between =blkcg_bio_issue_check() and cgroup_rmdir()").
quoted hunk ↗ jump to hunk
Cc: stable@vger.kernel.org --- block/cfq-iosched.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) =20diff --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;
=20
for (i =3D 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] =3D NULL;
+ }
+ if (cfqg->async_cfqq[1][i]) {
cfq_put_queue(cfqg->async_cfqq[1][i]);
+ cfqg->async_cfqq[1][i] =3D NULL;
+ }
}
=20
- if (cfqg->async_idle_cfqq)
+ if (cfqg->async_idle_cfqq) {
cfq_put_queue(cfqg->async_idle_cfqq);
+ cfqg->async_idle_cfqq =3D NULL;
+ }
=20
/*
* @blkg is going offline and will be ignored by