Re: [PATCH V4 1/7] blk-mq: grab .q_usage_counter when queuing request from plug code path

[Ming Lei <ming.lei@redhat.com>]

On Fri, Apr 05, 2019 at 05:26:24PM +0800, Dongli Zhang wrote:
> Hi Ming,
> 
> On 04/04/2019 04:43 PM, Ming Lei wrote:
> > Just like aio/io_uring, we need to grab 2 refcount for queuing one
> > request, one is for submission, another is for completion.
> > 
> > If the request isn't queued from plug code path, the refcount grabbed
> > in generic_make_request() serves for submission. In theroy, this
> > refcount should have been released after the sumission(async run queue)
> > is done. blk_freeze_queue() works with blk_sync_queue() together
> > for avoiding race between cleanup queue and IO submission, given async
> > run queue activities are canceled because hctx->run_work is scheduled with
> > the refcount held, so it is fine to not hold the refcount when
> > running the run queue work function for dispatch IO.
> > 
> > However, if request is staggered into plug list, and finally queued
> > from plug code path, the refcount in submission side is actually missed.
> > And we may start to run queue after queue is removed because the queue's
> > kobject refcount isn't guaranteed to be grabbed in flushing plug list
> > context, then kernel oops is triggered, see the following race:
> > 
> > blk_mq_flush_plug_list():
> >         blk_mq_sched_insert_requests()
> >                 insert requests to sw queue or scheduler queue
> >                 blk_mq_run_hw_queue
> > 
> > Because of concurrent run queue, all requests inserted above may be
> > completed before calling the above blk_mq_run_hw_queue. Then queue can
> > be freed during the above blk_mq_run_hw_queue().
> > 
> > Fixes the issue by grab .q_usage_counter before calling
> > blk_mq_sched_insert_requests() in blk_mq_flush_plug_list(). This way is
> > safe because the queue is absolutely alive before inserting request.
> > 
> > Cc: Dongli Zhang <dongli.zhang@oracle.com>
> > Cc: James Smart <james.smart@broadcom.com>
> > Cc: Bart Van Assche <bart.vanassche@wdc.com>
> > Cc: linux-scsi@vger.kernel.org,
> > Cc: Martin K . Petersen <martin.petersen@oracle.com>,
> > Cc: Christoph Hellwig <hch@lst.de>,
> > Cc: James E . J . Bottomley <jejb@linux.vnet.ibm.com>,
> > Cc: jianchao wang <jianchao.w.wang@oracle.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 3ff3d7b49969..5b586affee09 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -1728,9 +1728,12 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
> >  		if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx) {
> >  			if (this_hctx) {
> >  				trace_block_unplug(this_q, depth, !from_schedule);
> > +
> > +				percpu_ref_get(&this_q->q_usage_counter);
> 
> Sorry to bother but I would just like to double confirm the reason to use
> "percpu_ref_get()" here which does not check whether the queue has been frozen.
> 
> Is it because there is assumption that any direct/indirect caller of
> blk_mq_flush_plug_list() much have already grabbed q_usage_counter, which is
> similar to blk_queue_enter_live()?

Because there is request in the plug list to be queued.

Thanks,
Ming