Re: [PATCH 1/6] io_uring: further limit non-owner defer-tw cq waiting

[Dylan Yudaken <dylany@fb.com>]

On Thu, 2022-09-08 at 16:56 +0100, Pavel Begunkov wrote:
> In case of DEFER_TASK_WORK we try to restrict waiters to only one
> task,
> which is also the only submitter; however, we don't do it reliably,
> which might be very confusing and backfire in the future. E.g. we
> currently allow multiple tasks in io_iopoll_check().
> 
> Fixes: dacbb30102689 ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  io_uring/io_uring.c |  6 ++++++
>  io_uring/io_uring.h | 11 +++++++++++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 0482087b7c64..dc6f64ecd926 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -1398,6 +1398,9 @@ static int io_iopoll_check(struct io_ring_ctx
> *ctx, long min)
>         int ret = 0;
>         unsigned long check_cq;
>  
> +       if (!io_allowed_run_tw(ctx))
> +               return -EEXIST;
> +
>         check_cq = READ_ONCE(ctx->check_cq);
>         if (unlikely(check_cq)) {
>                 if (check_cq & BIT(IO_CHECK_CQ_OVERFLOW_BIT))
> @@ -2386,6 +2389,9 @@ static int io_cqring_wait(struct io_ring_ctx
> *ctx, int min_events,
>         ktime_t timeout = KTIME_MAX;
>         int ret;
>  
> +       if (!io_allowed_run_tw(ctx))
> +               return -EEXIST;
> +
>         do {
>                 /* always run at least 1 task work to process local
> work */
>                 ret = io_run_task_work_ctx(ctx);
> diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
> index 9d89425292b7..4eea0836170e 100644
> --- a/io_uring/io_uring.h
> +++ b/io_uring/io_uring.h
> @@ -329,4 +329,15 @@ static inline struct io_kiocb
> *io_alloc_req(struct io_ring_ctx *ctx)
>         return container_of(node, struct io_kiocb, comp_list);
>  }
>  
> +static inline bool io_allowed_run_tw(struct io_ring_ctx *ctx)
> +{
> +       if (!(ctx->flags & IORING_SETUP_DEFER_TASKRUN))
> +               return true;
> +       if (unlikely(ctx->submitter_task != current)) {
> +               /* maybe this is before any submissions */
> +               return !ctx->submitter_task;
> +       }
> +       return true;
> +}
> +
>  #endif

Reviewed-by: Dylan Yudaken <dylany@fb.com>