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SubjectRe: Block regression since 3.1-rc3
Hello, Mike.

On Tue, Oct 11, 2011 at 03:56:12PM -0400, Mike Snitzer wrote:
> > I don't object to the immediate fix but think that adding such special
> > case is gonna make the thing even more brittle and make future changes
> > even more difficult. Those one off cases tend to cause pretty severe
> > headache when someone wants to evolve common code, so let's please
> > find out what went wrong and fix it properly so that everyone follows
> > the same set of rules.
>
> Are you referring to Jeff's fix as "the immediate fix"? Christophe
> seems to have had success with it after all.

I meant reverting the previous commit. Oops... it seems like I
misread Jeff's patch. Please read on.

> As for the special case that you're suggesting makes the code more
> brittle, etc. If you could be more specific that'd be awesome.

I was still talking about the previous attempt of making dm treated
special by flush machinery. (the purity thing someone was talking
about)

> Jeff asked a question about the need to kick the queue in this case (as
> he didn't feel he had a proper justification for why it was needed).
>
> If we can get a proper patch header together to justify Jeff's patch
> that'd be great. And then revisit any of the special casing you'd like
> us to avoid in >= 3.2?
>
> (we're obviously _very_ short on time for a 3.1 fix right now).
...
> > Hmmm... another rather nasty assumption the current flush code makes
> > is that every flush request has either zero or single bio attached to
> > it. The assumption has always been there for quite some time now.
>
> OK.
>
> > That somehow seems broken by request based dm (either that or wrong
> > request is taking INSERT_FLUSH path).
>
> Where was this issue of a flush having multiple bios reported?

I was misreading Jeff's patch, so the problem is request w/o bio
reaching INSERT_FLUSH, not rq's with multiple bio's. Sorry about
that. Having another look...

Ah, okay, so, blk-flush on the lower layer device is seeing
q->flush_rq of the upper layer which doesn't have bio. Yes, the
BUG_ON() change looks correct to me. That or we can do

BUG_ON(rq->bio != rq->bio_tail); /* assumes zero or single bio rq */

As for the blk_run_queue_async(), it's a bit confusing. Currently,
the block layer isn't clear about who's responsible kicking the queue
after putting a request onto elevator and I suppose Jeff put it there
because blk_insert_cloned_request() doesn't kick the queue.

Hmm... Jeff, you also added blk_run_queue_async() call in
4853abaae7e4a too. Is there a reason why blk_insert_cloned_request()
isn't calling __blk_run_queue() or async variant of it like
blk_insert_request() does?

At any rate, the queue kicking is a different issue. Let's not mix
the two here. The BUG_ON() change looks good to me.

Thank you.

--
tejun


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