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

[Paolo Valente]

> 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