Re: [PATCH] scsi: core: fix race between handling STS_RESOURCE and completion

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

On Wed, Dec 02, 2020 at 10:10:30AM -0800, Bart Van Assche wrote:
> On 12/2/20 2:04 AM, Ming Lei wrote:
> > When queuing IO request to LLD, STS_RESOURCE may be returned because:
> > 
> > - host in recovery or blocked
> > - target queue throttling or blocked
> > - LLD rejection
> > 
> > Any one of the above doesn't happen frequently enough.
> > 
> > BLK_STS_DEV_RESOURCE is returned to block layer for avoiding unnecessary
> > re-run queue, and it is just one small optimization. However, all
> > in-flight requests originated from this scsi device may be completed
> > just after reading 'sdev->device_busy', so BLK_STS_DEV_RESOURCE is
> > returned to block layer. And the current failed IO won't get chance
> > to be queued any more, since it is invisible at that time for either
> > scsi_run_queue_async() or blk-mq's RESTART.
> > 
> > Fix the issue by not returning BLK_STS_DEV_RESOURCE in this situation.
> > 
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Sumit Saxena <sumit.saxena@broadcom.com>
> > Cc: Kashyap Desai <kashyap.desai@broadcom.com>
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Ewan Milne <emilne@redhat.com>
> > Cc: Long Li <longli@microsoft.com>
> > Tested-by: "chenxiang (M)" <chenxiang66@hisilicon.com>
> > Reported-by: John Garry <john.garry@huawei.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   drivers/scsi/scsi_lib.c | 3 +--
> >   1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index 60c7a7d74852..03c6d0620bfd 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -1703,8 +1703,7 @@ static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
> >   		break;
> >   	case BLK_STS_RESOURCE:
> >   	case BLK_STS_ZONE_RESOURCE:
> > -		if (atomic_read(&sdev->device_busy) ||
> > -		    scsi_device_blocked(sdev))
> > +		if (scsi_device_blocked(sdev))
> >   			ret = BLK_STS_DEV_RESOURCE;
> >   		break;
> >   	default:
> 
> Since this patch modifies code introduced in commit 86ff7c2a80cd ("blk-mq:
> introduce BLK_STS_DEV_RESOURCE"), does this patch perhaps needs a Fixes:
> tag?

This same race exists before commit 86ff7c2a80cd, so I think the 'Fixes:' tag
is misleading.



Thanks,
Ming