[Mesa-dev] [PATCH] vulkan/wsi: Avoid waiting indefinitely for present completion in x11_manage_fifo_queues().

[Henri Verbeet]

On 24 October 2017 at 20:13, Fredrik Höglund <fredrik at kde.org> wrote:
> On Tuesday 24 October 2017, Henri Verbeet wrote:
>> On 24 October 2017 at 16:11, Fredrik Höglund <fredrik at kde.org> wrote:
>> >> @@ -934,9 +938,18 @@ x11_manage_fifo_queues(void *state)
>> >>
>> >>        while (chain->last_present_msc < target_msc) {
>> >>           xcb_generic_event_t *event =
>> >> -            xcb_wait_for_special_event(chain->conn, chain->special_event);
>> >> -         if (!event)
>> >> -            goto fail;
>> >> +            xcb_poll_for_special_event(chain->conn, chain->special_event);
>> >> +         if (!event) {
>> >> +            int ret = poll(&pfds, 1, 100);
>> >
>> > There is a race condition here where another thread can read the event
>> > from the file descriptor in the time between the calls to
>> > xcb_poll_for_special_event() and poll().
>> >
>> Is that a scenario we care about? Unless I'm misunderstanding
>> something, the same kind of thing could happen between
>> x11_present_to_x11() and xcb_wait_for_special_event().
>
> It cannot, because if another thread reads a special event, xcb will
> insert it into the corresponding special event queue and wake the
> waiting thread. That's the point of having special event queues.
>
> But the reason I know this to be a problem is that I have tried to fix
> this bug in the same way, and I noticed that it resulted in frequent
> random stutters in some apps because poll() was timing out.
>
Oh, I think I see what you mean. The event wouldn't get lost, but we'd
have to wait for the poll to timeout to get to it because it's already
read from the fd into the queue.