[PATCH wayland v2] event-loop: Dispatch idle callbacks twice

[Giulio Camuffo]

2015-01-28 17:49 GMT+02:00 Derek Foreman <derekf at osg.samsung.com>:
> On 28/01/15 09:39 AM, Giulio Camuffo wrote:
>> 2015-01-28 17:25 GMT+02:00 Derek Foreman <derekf at osg.samsung.com>:
>>> To fix a shutdown crash in weston's x11 compositor I want to move the
>>> weston X window close to an idle handler.
>>>
>>> Since idle handlers are processed at the start of an event loop, the
>>> handler that deals with window close will run at the start of the
>>> next input_loop dispatch, after which the dispatcher blocks on epoll
>>> forever (since all input events that will ever occur have been consumed).
>>>
>>> Dispatching idle callbacks both at the start and end of event-loop
>>> processing will prevent this permanent blocking.
>>>
>>> Note that just moving the callback dispatch could theoretically
>>> result in an idle callback being delayed indefinitely while waiting
>>> for epoll_wait() to complete.
>>>
>>> Callbacks are removed from the list when they're run, so the second
>>> dispatch won't result in any extra calls.
>>>
>>> Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
>>> ---
>>>
>>> Amusingly, the problem Bill points out is exactly the problem this patch is
>>> trying to correct.
>>>
>>> This version of the patch leaves the original dispatch and adds a second.
>>>
>>>
>>>  src/event-loop.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/src/event-loop.c b/src/event-loop.c
>>> index 1f571ba..d257d78 100644
>>> --- a/src/event-loop.c
>>> +++ b/src/event-loop.c
>>> @@ -421,10 +421,12 @@ wl_event_loop_dispatch(struct wl_event_loop *loop, int timeout)
>>>
>>>         wl_event_loop_process_destroy_list(loop);
>>>
>>> +       wl_event_loop_dispatch_idle(loop);
>>> +
>>
>> How does this fix the problem? The idle callbacks are still called
>> before the epoll_wait(), so shouldn't the problem you describe in the
>> commit message still happen?
>> Besides, the idle callbacks are anyway going to be called very soon at
>> the next loop in wl_display_run() after wl_event_loop_dispatch()
>> returns. Or is the post_dispatch_check() call below involved somehow?
>
> The callback in question is added by the handler
> source->interface->dispatch() calls in the middle.  :)
>
> So the second idle dispatch gets it, and the loop isn't entered again.
>
> Without the change it won't get called until the loop is entered, and
> there's no events left to process, so epoll_wait() just sits...

Ah, so the loop is broken and wl_display_run() returns after the
current iteration. Looks good then.

>
>> --
>> Giulio
>>
>>>         do {
>>>                 n = post_dispatch_check(loop);
>>>         } while (n > 0);
>>> -
>>> +
>>>         return 0;
>>>  }
>>>
>>> --
>>> 2.1.4
>>>
>>> _______________________________________________
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>>> wayland-devel at lists.freedesktop.org
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>