Re: dm: fix free_rq_clone() NULL pointer when requeueing unmapped request

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Apr 30 2015 at  5:07am -0400,
Bart Van Assche <bart.vanassche@sandisk.com> wrote:

> On 04/29/15 21:53, Mike Snitzer wrote:
> >On Wed, Apr 29 2015 at  3:11P -0400,
> >Bart Van Assche <bart.vanassche@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@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@sandisk.com>
> >Signed-off-by: Mike Snitzer <snitzer@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.

Great.

> Thanks !

No problem, thanks for testing.