[RFC PATCH 3/3] blk-mq: Remove generation seqeunce

[keith.busch@linux.intel.com (Keith Busch)]

On Wed, Jul 18, 2018@10:39:36PM +0200, hch@lst.de wrote:
> On Wed, Jul 18, 2018@09:56:50PM +0200, hch@lst.de wrote:
> > On Fri, Jul 13, 2018@05:58:08PM -0600, Keith Busch wrote:
> > > Of the two you mentioned, yours is preferable IMO. While I appreciate
> > > Jianchao's detailed analysis, it's hard to take a proposal seriously
> > > that so colourfully calls everyone else "dangerous" while advocating
> > > for silently losing requests on purpose.
> > > 
> > > But where's the option that fixes scsi to handle hardware completions
> > > concurrently with arbitrary timeout software? Propping up that house of
> > > cards can't be the only recourse.
> > 
> > The important bit is that we need to fix this issue quickly.  We are
> > past -rc5 so I'm rather concerned about anything too complicated.
> > 
> > I'm not even sure SCSI has a problem with multiple completions happening
> > at the same time, but it certainly has a problem with bypassing
> > blk_mq_complete_request from the EH path.
> > 
> > I think we can solve this properly, but I also think we are way to late
> > in the 4.18 cycle to fix it properly.  For now I fear we'll just have
> > to revert the changes and try again for 4.19 or even 4.20 if we don't
> > act quickly enough.
> 
> So here is a quick attempt at the revert while also trying to keep
> nvme working.  Keith, Bart, Jianchao - does this looks reasonable
> as a 4.18 band aid?
> 
> http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/blk-eh-revert

Hm, I not really a fan. The far majority of blk-mq drivers don't even
implement a timeout, and reverting it will lose their requests forever
if they complete at the same time as a timeout.

Of the remaining drivers, most of those don't want the reverted behavior
either. It actually looks like scsi is the only mq driver that wants
to block completions. In the short term, scsi can make that happen with
just three lines of code.

---
diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
index 8932ae81a15a..03986af3076c 100644
--- a/drivers/scsi/scsi_error.c
+++ b/drivers/scsi/scsi_error.c
@@ -286,6 +286,10 @@ enum blk_eh_timer_return scsi_times_out(struct request *req)
 	enum blk_eh_timer_return rtn = BLK_EH_DONE;
 	struct Scsi_Host *host = scmd->device->host;

+	if (req->q->mq_ops && cmpxchg(&rq->state, MQ_RQ_IN_FLIGHT, MQ_RQ_COMPLETE) !=
+							MQ_RQ_IN_FLIGHT);
+		return rtn;
+
	trace_scsi_dispatch_cmd_timeout(scmd);
	scsi_log_completion(scmd, TIMEOUT_ERROR);


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