On Thu, Oct 15 2015 at 11:08pm -0400, Ming Lei <tom.leiming@xxxxxxxxx> wrote: > On Thu, Oct 15, 2015 at 4:06 PM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > > On Wed, Oct 14 2015 at 11:27pm -0400, > > Ming Lei <tom.leiming@xxxxxxxxx> wrote: > > > >> On Sat, Oct 10, 2015 at 3:52 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > >> > > >> > Turns out that this change: > >> > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=2639638c77768a86216be456c2764e32a2bcd841 > >> > > >> > needed to be reverted with: > >> > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=ad3ccd760da7c05b90775372f9b39dc2964086fe > >> > > >> > Because nested plugs caused generic_make_request()'s onstack bio_list to > >> > go out of scope (blk_finish_plug() wouldn't actually flush the list > >> > within generic_make_request because XFS already added an outermost > >> > plug). > >> > >> Looks you should have defined bio_list in plug as > >> > >> 'struct bio_list bio_list' > >> > >> instead of one pointer. > > > > I realized that and fixed it (see commit ad3ccd760da7c05b90 referenced > > above that does exactly that). That wasn't the problem. > > OK. > > > > >> > > >> > But even after fixing that I then hit issues with these changes now > >> > resulting in imperfect 'in_generic_make_request' accounting that happens > >> > lazily once the outermost plug completes blk_finish_plug. manifested as > >> > dm-bufio.c:dm_bufio_prefetch's BUG_ON(dm_bufio_in_request()); hitting. > >> > >> Looks this problem should be related with above 'bio_list' definition too. > > > > No, as I explained it was due to the nested plug: > > > >> > > >> > Basically using the blk-core's onstack plugging isn't workable for > >> > fixing this deadlock and we're back to having to seriously consider > >> > this (with its additional hook in the scheduler) > > > > To elaborate, for the code in DM (and other subsystems like bcache) that > > rely on accurate accounting of whether we're actively _in_ > > generic_make_request: using plug to store/manage the bio_list isn't > > That looks an interesting requirement, which means DM just need to know > if the current callsite is from generic_make_request(), so what you need > is just one per-task variable. > > With the stack variable of 'plug', it should be easier to do that for DM, for > example, you can introduce one flag in 'struct blk_plug', then set it in > the entry of generic_make_request(), and clear it in the exit of the > function. Yes, I mean we _could_ set/clear the 'in_generic_make_request' flag _in_ generic_make_request() but then it just calls into question why the heck we're using the plug to begin with? (especially given plugging is for request-based devices at this point!). It really doesn't make _any_ sense to overload blk_plug by moving the bio_list into there and adding a 'in_generic_make_request'... when you consider the _only_ reason this was suggested is to (ab)use the existing hook in scheduler/core.c. So I stand by my position that there is really no point in the exercise and that it actually hurts the code to try to make this a blk_plug "feature". We already have well established current->bio_list semantics that can be reused as a flag given it is a pointer. The block callout in the scheduler is going to grow a conditional either way. What I've proposed _seems_ the cleanest to me and others. Hopefully you can see that aspect of things. So if you could review the v3 patch with a critical eye that'd be very much appreciated. But I do look forward to Jens also having a look at this and providing his review feedback. > > workable because nested plugs change the lifetime of when the bio_list > > is processed (as I implemented it -- which was to respect nested plugs). > > I could've forced the issue by making the bio_list get processed > > regardless of nesting but that would've made the onstack plugging much > > more convoluted (duality between nested vs not just for bio_list's > > benefit and for what gain? Simply to avoid an extra conditional > > immediately in the scheduler? That conditional was still added anyway > > but just as part of blk_needs_flush_plug so in the end there wasn't any > > benefit!). > > > > Hopefully my middle-of-the-night reply is coherent and helped to clarify > > my position that (ab)using blk_plug for the bio_list management is > > _really_ awkward. ;) > > Hope it wan't my reply to cause the break of your sleep, :-) No, my dog woke me up to go outside at 4am.. I was up and couldn't resist looking at my phone.. the rest is history ;) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
