On 08/12/2020 21:10, Jens Axboe wrote: > On 12/8/20 12:24 PM, Pavel Begunkov wrote: >> On 08/12/2020 19:17, Jens Axboe wrote: >>> On 12/8/20 12:12 PM, Pavel Begunkov wrote: >>>> On 07/12/2020 16:28, Jens Axboe wrote: >>>>> On Sun, Dec 6, 2020 at 3:26 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote: >>>>>> From: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx> >>>>>> >>>>>> The reason is that once we got a non EAGAIN error in io_wq_submit_work(), >>>>>> we'll complete req by calling io_req_complete(), which will hold completion_lock >>>>>> to call io_commit_cqring(), but for polled io, io_iopoll_complete() won't >>>>>> hold completion_lock to call io_commit_cqring(), then there maybe concurrent >>>>>> access to ctx->defer_list, double free may happen. >>>>>> >>>>>> To fix this bug, we always let io_iopoll_complete() complete polled io. >>>>> >>>>> This patch is causing hangs with iopoll testing, if you end up getting >>>>> -EAGAIN on request submission. I've dropped it. >>>> >>>> I fail to understand without debugging how does it happen, especially since >>>> it shouldn't even get out of the while in io_wq_submit_work(). Is that >>>> something obvious I've missed? >>> >>> I didn't have time to look into it, and haven't yet, just reporting that >>> it very reliably fails (and under what conditions). >> >> Yeah, I get it, asked just in case. >> I'll see what's going on if Xiaoguang wouldn't handle it before. > > Should be trivial to reproduce on eg nvme by doing: > > echo mq-deadline > /sys/block/nvme0n1/queue/scheduler > echo 2 > /sys/block/nvme0n1/queue/nr_requests > > and then run test/iopoll on that device. I'll try and take a look > tomorrow unless someone beats me to it. Tried out with iopoll-enabled null_blk. test/iopoll fails with "test_io_uring_submit_enters failed", but if remove iteration limit from the test, it completes... eventually. premise: io_complete_rw_iopoll() gets -EAGAIN but returns 0 to io_wq_submit_work(). The old version happily completes IO with that 0, but the patch delays it to do_iopoll(), which retries and so all that repeats. And, I believe, that the behaviour that io_wq_submit_work()'s -EGAIN check was trying to achieve... The question left is why no one progresses. May even be something in block. Need to trace further. -- Pavel Begunkov
