On Fri, Feb 22 2019 at 9:02pm -0500, John Dorminy <jdorminy@xxxxxxxxxx> wrote: > I am perhaps not understanding the intricacies here, or not seeing a > barrier protecting it, so forgive me if I'm off base. I think reading > parent->bi_status here is unsafe. > Consider the following sequence of events on two threads. > > Thread 0 Thread 1 > In __bio_chain_endio: In __bio_chain_endio: > [A] Child 0 reads parent->bi_status, > no error. > Child bio 1 reads parent, no error seen > It sets parent->bi_status to an error > It calls bio_put. > Child bio 0 calls bio_put > [end __bio_chain_endio] [end __bio_chain_endio] > In bio_chain_endio(), bio_endio(parent) > is called, calling bio_remaining_done() > which decrements __bi_remaining to 1 > and returns false, so no further endio > stuff is done. > In bio_chain_endio(), bio_endio(parent) > is called, calling bio_remaining_done(), > decrementing parent->__bi_remaining to > 0, and continuing to finish parent. > Either for block tracing or for parent's > bi_end_io(), this thread tries to read > parent->bi_status again. > > The compiler or the CPU may cache the read from [A], and since there > are no intervening barriers, parent->bi_status is still believed on > thread 0 to be success. Thus the bio may still be falsely believed to > have completed successfully, even though child 1 set an error in it. > > Am I missing a subtlety here? Either neilb's original or even Jens' suggestion would be fine though. > if (!parent->bi_status && bio->bi_status) > parent->bi_status = bio->bi_status; Even if your scenario did play out (which I agree it looks possible) it'd just degenerate to neilb's version: > if (bio->bi_status) > parent->bi_status = bio->bi_status; Which also accomplishes fixing what Neil originally detailed in his patch header.
