> Il giorno 17 ago 2018, alle ore 19:28, Maciej S. Szmigiero <mail@xxxxxxxxxxxxxxxxxxxxx> ha scritto:
>
> 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.
>
By chance, did you check whether is BFQ is ok in this respect?
Thanks,
Paolo
> Signed-off-by: Maciej S. Szmigiero <mail@xxxxxxxxxxxxxxxxxxxxx>
> Fixes: 4c6994806f70 ("blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()").
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> 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
