[PATCH 6/8] block: drivers: complete request locally from blk_mq_tagset_busy_iter

[Ming Lei]

On Sun, Apr 25, 2021 at 08:02:10PM -0700, Bart Van Assche wrote:
> On 4/25/21 1:57 AM, Ming Lei wrote:
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index c289991ffaed..7cbaee282b6d 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -1568,7 +1568,11 @@ static void scsi_mq_done(struct scsi_cmnd *cmd)
> >  	if (unlikely(test_and_set_bit(SCMD_STATE_COMPLETE, &cmd->state)))
> >  		return;
> >  	trace_scsi_dispatch_cmd_done(cmd);
> > -	blk_mq_complete_request(cmd->request);
> > +
> > +	if (unlikely(host_byte(cmd->result) != DID_OK))
> > +		blk_mq_complete_request_locally(cmd->request);
> > +	else
> > +		blk_mq_complete_request(cmd->request);
> >  }
> 
> This change is so tricky that it deserves a comment.
> 
> An even better approach would be *not* to export
> blk_mq_complete_request_locally() from the block layer to block drivers
> and instead modify the block layer such that it completes a request on
> the same CPU if request completion happens from inside the context of a
> tag iteration function. That would save driver writers the trouble of
> learning yet another block layer API.

Yeah, that is possible, and one request flag(eg. RQF_ITERATED) can be added.
The flag is set before calling ->fn(), and evaluated in
blk_mq_complete_request_remote().

Thanks,
Ming