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. > 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, :-) Thanks, Ming Lei -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
