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