Tejun Heo <tj@xxxxxxxxxx> writes: > 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. I can submit a two part series, sure. I'll have to get back to you on where I think the right place for the queue kick is. I'll look at it in detail tomorrow. -Jeff -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
