On Wed, May 13 2015 at 10:49pm -0400, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > As you know commit c4cf5261 avoids the atomic operations during endio > unless BIO_CHAIN is set. It only gets set if bio_inc_remaining() is > used. But bio_inc_remaining() is only used _after_ the initial endio -- > we needed BIO_CHAIN set earlier for these cases. Seems we need a new > function to register the intent to cascade endios (to allow for proper > accounting)? > > We could then switch callers to something like: > > bio_set_chain(bio); // establishes BIO_CHAIN iff __bio_remaining is 1? > old = io->bi_end_io; > bio->bi_end_io = new; > > in new(): > bio->bi_end_io = old; > bio_inc_remaining(bio) > > Anyway, not sure about the proper fix -- I do know this commit is a > definite regression (but it does nicely avoid the costly atomics ;) Just adding this to dm-thin.c:save_and_set_endio() fixed it: bio->bi_flags |= (1 << BIO_CHAIN); (setting a bit doesn't need a bio_set_chain fn like I suggested above). But, all relevant callers would need this treatment (I'd imagine bio_endio_nodec callers need help too? -- either that or __bi_remaining is completely irrelevant for all old users except for bio_chain()). -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
