[vlc-devel] [PATCH 1/6] os2: implement vlc_once()

[Rémi Denis-Courmont]

Le lauantaina 7. huhtikuuta 2018, 19.06.07 EEST KO Myung-Hun a écrit :
> > At least a compiler barrier *in practice*, but not in theory (so this is
> > not guaranteed). And the CPU might also reorder.
> 
> Reordering of both a compiler and a CPU should guarantee that the result
> of reordering is the same as the one of the sequential in a single thread.

Sure, all the provided patches work fine in the single thread case on any 
platform.

But the whole point of vlc_once() is deal with multiple threads and concurrent 
calls.

> And compiler cannot see the internal of cb().

With LTO and IPA combined, a compiler can propagate vlc_CPU_init as the cb 
value into vlc_once() from vlc_CPU().

Not that it matters much. The problem is not a lack of release fence. 
Unlocking the mutex provides a release fence. The problem is the lack fo 
acquire fence before reading the flag.

-- 
雷米‧德尼-库尔蒙
https://www.remlab.net/