Re: [PATCH] kyber: Fix crash in kyber_finish_request()

[Omar Sandoval]

On Mon, Sep 07, 2020 at 10:41:16AM -0600, Jens Axboe wrote:
> CC Omar
> 
> On 9/7/20 1:43 AM, Yang Yang wrote:
> > Kernel crash when requeue flush request.
> > It can be reproduced as below:
> > 
> > [    2.517297] Unable to handle kernel paging request at virtual address ffffffd8071c0b00
> > ...
> > [    2.517468] pc : clear_bit+0x18/0x2c
> > [    2.517502] lr : sbitmap_queue_clear+0x40/0x228
> > [    2.517503] sp : ffffff800832bc60 pstate : 00c00145
> > ...
> > [    2.517599] Process ksoftirqd/5 (pid: 51, stack limit = 0xffffff8008328000)
> > [    2.517602] Call trace:
> > [    2.517606]  clear_bit+0x18/0x2c
> > [    2.517619]  kyber_finish_request+0x74/0x80
> > [    2.517627]  blk_mq_requeue_request+0x3c/0xc0
> > [    2.517637]  __scsi_queue_insert+0x11c/0x148
> > [    2.517640]  scsi_softirq_done+0x114/0x130
> > [    2.517643]  blk_done_softirq+0x7c/0xb0
> > [    2.517651]  __do_softirq+0x208/0x3bc
> > [    2.517657]  run_ksoftirqd+0x34/0x60
> > [    2.517663]  smpboot_thread_fn+0x1c4/0x2c0
> > [    2.517667]  kthread+0x110/0x120
> > [    2.517669]  ret_from_fork+0x10/0x18
> > 
> > Signed-off-by: Yang Yang <yang.yang@xxxxxxxx>
> > ---
> >  block/kyber-iosched.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/block/kyber-iosched.c b/block/kyber-iosched.c
> > index a38c5ab103d1..af73afe7a05c 100644
> > --- a/block/kyber-iosched.c
> > +++ b/block/kyber-iosched.c
> > @@ -611,6 +611,9 @@ static void kyber_finish_request(struct request *rq)
> >  {
> >  	struct kyber_queue_data *kqd = rq->q->elevator->elevator_data;
> >  
> > +	if (unlikely(!(rq->rq_flags & RQF_ELVPRIV)))
> > +		return;
> > +
> >  	rq_clear_domain_token(kqd, rq);
> >  }
> >  
> > 

It looks like BFQ also has this check. Wouldn't it make more sense to
check it in blk-mq, like we do for .finish_request() in
blk_mq_free_request()? Something along these lines:

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index c34b090178a9..fa98470df3f0 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5895,18 +5895,6 @@ static void bfq_finish_requeue_request(struct request *rq)
 	struct bfq_queue *bfqq = RQ_BFQQ(rq);
 	struct bfq_data *bfqd;
 
-	/*
-	 * Requeue and finish hooks are invoked in blk-mq without
-	 * checking whether the involved request is actually still
-	 * referenced in the scheduler. To handle this fact, the
-	 * following two checks make this function exit in case of
-	 * spurious invocations, for which there is nothing to do.
-	 *
-	 * First, check whether rq has nothing to do with an elevator.
-	 */
-	if (unlikely(!(rq->rq_flags & RQF_ELVPRIV)))
-		return;
-
 	/*
 	 * rq either is not associated with any icq, or is an already
 	 * requeued request that has not (yet) been re-inserted into
diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
index 126021fc3a11..e81ca1bf6e10 100644
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -66,7 +66,7 @@ static inline void blk_mq_sched_requeue_request(struct request *rq)
 	struct request_queue *q = rq->q;
 	struct elevator_queue *e = q->elevator;
 
-	if (e && e->type->ops.requeue_request)
+	if ((rq->rq_flags & RQF_ELVPRIV) && e && e->type->ops.requeue_request)
 		e->type->ops.requeue_request(rq);
 }