On 14/12/2020 15:49, Xiaoguang Wang wrote:
> io_iopoll_complete() does not hold completion_lock to complete polled
> io, so in io_wq_submit_work(), we can not call io_req_complete() directly,
> to complete polled io, otherwise there maybe concurrent access to cqring,
> defer_list, etc, which is not safe. Commit dad1b1242fd5 ("io_uring: always
> let io_iopoll_complete() complete polled io") has fixed this issue, but
> Pavel reported that IOPOLL apart from rw can do buf reg/unreg requests(
> IORING_OP_PROVIDE_BUFFERS or IORING_OP_REMOVE_BUFFERS), so the fix is
> not good.
>
> Given that io_iopoll_complete() is always called under uring_lock, so here
> for polled io, we can also get uring_lock to fix this issue.
One thing I don't like is that io_wq_submit_work() won't be able to
publish an event while someone polling io_uring_enter(ENTER_GETEVENTS),
that's because both take the lock. The problem is when the poller waits
for an event that is currently in io-wq (i.e. io_wq_submit_work()).
The polling loop will eventually exit, so that's not a deadlock, but
latency,etc. would be huge.
>
> Fixes: dad1b1242fd5 ("io_uring: always let io_iopoll_complete() complete polled io")
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
> ---
> fs/io_uring.c | 25 +++++++++++++++----------
> 1 file changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index f53356ced5ab..eab3d2b7d232 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6354,19 +6354,24 @@ static struct io_wq_work *io_wq_submit_work(struct io_wq_work *work)
> }
>
> if (ret) {
> + bool iopoll_enabled = req->ctx->flags & IORING_SETUP_IOPOLL;
> +
> /*
> - * io_iopoll_complete() does not hold completion_lock to complete
> - * polled io, so here for polled io, just mark it done and still let
> - * io_iopoll_complete() complete it.
> + * io_iopoll_complete() does not hold completion_lock to complete polled
> + * io, so here for polled io, we can not call io_req_complete() directly,
> + * otherwise there maybe concurrent access to cqring, defer_list, etc,
> + * which is not safe. Given that io_iopoll_complete() is always called
> + * under uring_lock, so here for polled io, we also get uring_lock to
> + * complete it.
> */
> - if (req->ctx->flags & IORING_SETUP_IOPOLL) {
> - struct kiocb *kiocb = &req->rw.kiocb;
> + if (iopoll_enabled)
> + mutex_lock(&req->ctx->uring_lock);
>
> - kiocb_done(kiocb, ret, NULL);
> - } else {
> - req_set_fail_links(req);
> - io_req_complete(req, ret);
> - }
> + req_set_fail_links(req);
> + io_req_complete(req, ret);
> +
> + if (iopoll_enabled)
> + mutex_unlock(&req->ctx->uring_lock);
> }
>
> return io_steal_work(req);
>
--
Pavel Begunkov
