[dm-devel] dm: fix free_rq_clone() NULL pointer when requeueing unmapped request

[Bart Van Assche]

On 04/29/15 21:53, Mike Snitzer wrote:
> On Wed, Apr 29 2015 at  3:11P -0400,
> Bart Van Assche <bart.vanassche at sandisk.com> wrote:
>
>> On 04/29/15 20:53, Mike Snitzer wrote:
>>> Actually, here is the proper 4.1-only fix (Bart please verify this works
>>> for you):
>>
>> Hello Mike,
>>
>> Thanks for the patch. But against which tree has this patch been generated ?
>> It doesn't seem to apply on v4.1-rc1:
>>
>> $ git reset --hard v4.1-rc1
>> HEAD is now at b787f68 Linux 4.1-rc1
>> $ patch -p1 < ~/\[PATCH\]\ dm\:\ fix\ free_rq_clone\(\)\ NULL\ pointer\
>> when\ requeueing\ unmapped\ request.eml
>> (Stripping trailing CRs from patch; use --binary to disable.)
>> patching file drivers/md/dm.c
>> Hunk #1 FAILED at 1031.
>> Hunk #2 succeeded at 1124 (offset 53 lines).
>> Hunk #3 succeeded at 1143 (offset 53 lines).
>> 1 out of 3 hunks FAILED -- saving rejects to file drivers/md/dm.c.rej
>
> It was implemented against my "private" wip2 branch (since rebased):
> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/log/?h=wip2
>
> Anyway, here it is rebased to 4.1-rc1 (BTW, I'm open to dropping the
> WARN_ON_ONCE but I need to research further.. if you guys think that
> there are perfectly resonable ways to explain why clone->q is NULL in
> the IO completion path then I'm all ears):
>
> From: Mike Snitzer <snitzer at redhat.com>
> Date: Wed, 29 Apr 2015 10:48:09 -0400
> Subject: dm: fix free_rq_clone() NULL pointer when requeueing unmapped request
>
> Commit 022333427a ("dm: optimize dm_mq_queue_rq to _not_ use kthread if
> using pure blk-mq") mistakenly removed free_rq_clone()'s clone->q check
> before testing clone->q->mq_ops.  It was an oversight to discontinue
> that check for 1 of the 2 use-cases for free_rq_clone():
> 1) free_rq_clone() called when an unmapped original request is requeued
> 2) free_rq_clone() called in the request-based IO completion path
>
> The clone->q check made sense for case #1 but not for #2.  However, we
> cannot just reinstate the check as it'd mask a serious bug in the IO
> completion case #2 -- no in-flight request should have an uninitialized
> request_queue (basic block layer refcounting _should_ ensure this).
>
> The NULL pointer seen for case #1 is detailed here:
> https://www.redhat.com/archives/dm-devel/2015-April/msg00160.html
>
> Fix this free_rq_clone() NULL pointer by simply checking if the
> mapped_device's type is DM_TYPE_MQ_REQUEST_BASED (clone's queue is
> blk-mq) rather than checking clone->q->mq_ops.  This avoids the need to
> dereference clone->q, but a WARN_ON_ONCE is added to let us know if an
> uninitialized clone request is being completed.
>
> Reported-by: Bart Van Assche <bart.vanassche at sandisk.com>
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> ---
>   drivers/md/dm.c | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 6754bbd..dfb7bde 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1082,18 +1082,26 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
>   	dm_put(md);
>   }
>
> -static void free_rq_clone(struct request *clone)
> +static void free_rq_clone(struct request *clone, bool must_be_mapped)
>   {
>   	struct dm_rq_target_io *tio = clone->end_io_data;
>   	struct mapped_device *md = tio->md;
>
> +	WARN_ON_ONCE(must_be_mapped && !clone->q);
> +
>   	blk_rq_unprep_clone(clone);
>
> -	if (clone->q->mq_ops)
> +	if (md->type == DM_TYPE_MQ_REQUEST_BASED)
> +		/* stacked on blk-mq queue(s) */
>   		tio->ti->type->release_clone_rq(clone);
>   	else if (!md->queue->mq_ops)
>   		/* request_fn queue stacked on request_fn queue(s) */
>   		free_clone_request(md, clone);
> +	/*
> +	 * NOTE: for the blk-mq queue stacked on request_fn queue(s) case:
> +	 * no need to call free_clone_request() because we leverage blk-mq by
> +	 * allocating the clone at the end of the blk-mq pdu (see: clone_rq)
> +	 */
>
>   	if (!md->queue->mq_ops)
>   		free_rq_tio(tio);
> @@ -1124,7 +1132,7 @@ static void dm_end_request(struct request *clone, int error)
>   			rq->sense_len = clone->sense_len;
>   	}
>
> -	free_rq_clone(clone);
> +	free_rq_clone(clone, true);
>   	if (!rq->q->mq_ops)
>   		blk_end_request_all(rq, error);
>   	else
> @@ -1143,7 +1151,7 @@ static void dm_unprep_request(struct request *rq)
>   	}
>
>   	if (clone)
> -		free_rq_clone(clone);
> +		free_rq_clone(clone, false);
>   }
>
>   /*

Hello Mike,

This patch survives my SRP initiator tests without triggering any kernel 
warning.

Thanks !

Bart.