On 4/21/21 4:19 PM, Hao Xu wrote:
> do this to avoid race below:
>
> userspace kernel
>
> | check sqring and iopoll_list
> submit sqe |
> check IORING_SQ_NEED_WAKEUP |
> (which is not set) | |
> | set IORING_SQ_NEED_WAKEUP
> wait cqe | schedule(never wakeup again)
Agree, the flag should be set first.
Haven't tried it, but the patch looks reasonable
>
> Signed-off-by: Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>
> ---
>
> Hi all,
> I'm doing some work to reduce cpu usage in low IO pression, and I
> removed timeout logic in io_sq_thread() to do some test with fio-3.26,
> I found that fio hangs in getevents, inifinitely trying to get a cqe,
> While sq-thread is sleeping. It seems there is race situation, and it
> is still there even after I fix the issue described above in the commit
> message. I doubt it is something to do with memory barrier logic
> between userspace and kernel, I'm trying to address it, not many clues
> for now.
> I'll send the fio config and kernel modification I did for test in
> following mail soon.
>
> Thanks,
> Hao
>
> fs/io_uring.c | 36 +++++++++++++++++++-----------------
> 1 file changed, 19 insertions(+), 17 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index dff34975d86b..042f1149db51 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -6802,27 +6802,29 @@ static int io_sq_thread(void *data)
> continue;
> }
>
> - needs_sched = true;
> prepare_to_wait(&sqd->wait, &wait, TASK_INTERRUPTIBLE);
> - list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
> - if ((ctx->flags & IORING_SETUP_IOPOLL) &&
> - !list_empty_careful(&ctx->iopoll_list)) {
> - needs_sched = false;
> - break;
> - }
> - if (io_sqring_entries(ctx)) {
> - needs_sched = false;
> - break;
> - }
> - }
> -
> - if (needs_sched && !test_bit(IO_SQ_THREAD_SHOULD_PARK, &sqd->state)) {
> + if (!test_bit(IO_SQ_THREAD_SHOULD_PARK, &sqd->state)) {
> list_for_each_entry(ctx, &sqd->ctx_list, sqd_list)
> io_ring_set_wakeup_flag(ctx);
>
> - mutex_unlock(&sqd->lock);
> - schedule();
> - mutex_lock(&sqd->lock);
> + needs_sched = true;
> + list_for_each_entry(ctx, &sqd->ctx_list, sqd_list) {
> + if ((ctx->flags & IORING_SETUP_IOPOLL) &&
> + !list_empty_careful(&ctx->iopoll_list)) {
> + needs_sched = false;
> + break;
> + }
> + if (io_sqring_entries(ctx)) {
> + needs_sched = false;
> + break;
> + }
> + }
> +
> + if (needs_sched) {
> + mutex_unlock(&sqd->lock);
> + schedule();
> + mutex_lock(&sqd->lock);
> + }
> list_for_each_entry(ctx, &sqd->ctx_list, sqd_list)
> io_ring_clear_wakeup_flag(ctx);
> }
>
--
Pavel Begunkov
