On Sat, Aug 08, 2020 at 12:34:39PM -0600, Jens Axboe wrote:
> An earlier commit:
>
> b7db41c9e03b ("io_uring: fix regression with always ignoring signals in io_cqring_wait()")
>
> ensured that we didn't get stuck waiting for eventfd reads when it's
> registered with the io_uring ring for event notification, but we still
> have a gap where the task can be waiting on other events in the kernel
> and need a bigger nudge to make forward progress.
>
> Ensure that we use signaled notifications for a task that isn't currently
> running, to be certain the work is seen and processed immediately.
>
> Cc: stable@xxxxxxxxxxxxxxx # v5.7+
> Reported-by: Josef <josef.grieb@xxxxxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> ---
> fs/io_uring.c | 22 ++++++++++++++--------
> 1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index e9b27cdaa735..443eecdfeda9 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1712,21 +1712,27 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb)
> struct io_ring_ctx *ctx = req->ctx;
> int ret, notify = TWA_RESUME;
>
> + ret = __task_work_add(tsk, cb);
> + if (unlikely(ret))
> + return ret;
> +
> /*
> * SQPOLL kernel thread doesn't need notification, just a wakeup.
> - * If we're not using an eventfd, then TWA_RESUME is always fine,
> - * as we won't have dependencies between request completions for
> - * other kernel wait conditions.
> + * For any other work, use signaled wakeups if the task isn't
> + * running to avoid dependencies between tasks or threads. If
> + * the issuing task is currently waiting in the kernel on a thread,
> + * and same thread is waiting for a completion event, then we need
> + * to ensure that the issuing task processes task_work. TWA_SIGNAL
> + * is needed for that.
> */
> if (ctx->flags & IORING_SETUP_SQPOLL)
> notify = 0;
> - else if (ctx->cq_ev_fd)
> + else if (READ_ONCE(tsk->state) != TASK_RUNNING)
> notify = TWA_SIGNAL;
>
> - ret = task_work_add(tsk, cb, notify);
> - if (!ret)
> - wake_up_process(tsk);
> - return ret;
> + __task_work_notify(tsk, notify);
> + wake_up_process(tsk);
> + return 0;
> }
Wait.. so the only change here is that you look at tsk->state, _after_
doing __task_work_add(), but nothing, not the Changelog nor the comment
explains this.
So you're relying on __task_work_add() being an smp_mb() vs the add, and
you order this against the smp_mb() in set_current_state() ?
This really needs spelling out.
