RE: +AFs-PATCH+AF0- blk-mq: Set request mapping to NULL in blk+AF8-mq+AF8-put+AF8-driver+AF8-tag

[Kashyap Desai <kashyap.desai@broadcom.com>]

> > On Thu, Dec 06, 2018 at 11:15:13AM +0530, Kashyap Desai wrote:
> > > >
> > > > If the 'tag' passed to scsi_host_find_tag() is valid, I think
there
> > > > shouldn't have such issue.
> > > >
> > > > If you want to find outstanding IOs, maybe you can try
> > > > blk_mq_queue_tag_busy_iter()
> > > > or blk_mq_tagset_busy_iter(), because you may not know if the
passed
> > > 'tag'
> > > > to
> > > > scsi_host_find_tag() is valid or not.
> > >
> > > We tried quick change in mpt3sas driver using
blk_mq_tagset_busy_iter
> and
> > > it returns/callback for valid requests (no stale entries are
returned).
> > > Expected.
> > > Above two APIs are only for blk-mq.  What about non-mq case ? Driver
> > > should use scsi_host_find_tag for non-mq and blk_mq_tagset_busy_iter
> for
> > > blk-mq case ?
> >
> > But your patch is only for blk-mq, is there same issue on non-mq case?
>
> Problematic part from below function is code path which goes from "
> shost_use_blk_mq(shost))".
> Non-mq path works fine because every IO completion set
bqt->tag_index[tag]
> = NULL from blk_queue_end_tag().
>
> I did similar things for mq path in this patch.
>
> static inline struct scsi_cmnd *scsi_host_find_tag(struct Scsi_Host
*shost,
>                 int tag)
> {
>         struct request *req = NULL;
>
>         if (tag == SCSI_NO_TAG)
>                 return NULL;
>
>         if (shost_use_blk_mq(shost)) {
>                 u16 hwq = blk_mq_unique_tag_to_hwq(tag);
>
>                 if (hwq < shost->tag_set.nr_hw_queues) {
>                         req = blk_mq_tag_to_rq(shost->tag_set.tags[hwq],
>                                 blk_mq_unique_tag_to_tag(tag));
>                 }
>         } else {
>                 req = blk_map_queue_find_tag(shost->bqt, tag);
>         }


Hi Jens,

Any conclusion/feedback on this topic/patch ?  As discussed, This is a
safe change and  good to have if no design issue.

Kashyap