[Xcb] [PATCH] Fix a thread hang with xcb_wait_for_special_event()

[Uli Schlachter]

Hi,

Am 15.06.2015 um 12:03 schrieb Michel Dänzer:
> On 12.06.2015 22:21, Uli Schlachter wrote:
[...]
>> +static void insert_special(special_list **prev_special, special_list *special, xcb_special_event_t *se)
>> +{
>> +    special->se = se;
>> +    special->next = *prev_special;
>> +    *prev_special = special;
>> +}
> 
> AFAICT this inserts the new special_list entry at the beginning of the
> linked list, so the thread corresponding to the last inserted entry will
> be woken up first, right? It might be fairer to make this a FIFO instead
> of a LIFO.
[...]

Hm. Yes, you are right. However, changing this either requires traversing the
linked list or adding a pointer to its tail. I don't really like either solution.

This doesn't really matter for different xcb_special_event_t instances (we can't
predict which one gets an event next anyway and event_special() will wake up the
right one). If two threads call xcb_wait_for_special_event() on the same
xcb_special_event_t instance, then there is a race anyway and I don't really see
why we should guarantee anything at all. Also, I can't really think of any
reason to call xcb_wait_for_special_event() concurrently.

So I'm not really convinced we need some kind of fairness.

What do you/others say?

Cheers,
Uli
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 - Vincent Ebert