[vlc-devel] [PATCH 1/6] os2: implement vlc_once()
komh78 at gmail.com
Sat Apr 7 18:06:07 CEST 2018
Romain Vimont wrote:
> On Fri, Apr 06, 2018 at 09:12:04PM +0900, KO Myung-Hun wrote:
>> Romain Vimont wrote:
>>> On Thu, Apr 05, 2018 at 08:40:57PM +0900, KO Myung-Hun wrote:
>>>> And if it is 1, it's guaranteed that cb() has been completed. Because
>>>> function call acts as memory barrier and it will not be reordered
>>> At least in theory, I think that a function call does not act as a
>>> memory barrier.
>> Ok, at least compiler barrier according to your link.
> 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.
However, if moving cb() around, then it cannot be guaranteed that they
are same. For example, cb() may depend on once->done in its internal.
And compiler cannot see the internal of cb(). As a result, compiler
should not reorder those parts, and neither CPU.
>>> And even if it did, so that cb() is guaranteed to be completed in the
>>> first thread, the changes applied by cb() are not guaranteed to be
>>> visible from the second thread (cache coherence).
>> For the second thread to use the changes by cb(), it should see done
>> first. And it can see done with 1, it should be able to see the changes
>> by cb(). After all, is it possible that changes of cb are not applied
>> but done is applied to each cache ?
> Yes, there is no guarantee that the second thread sees the changes
> applied by cb() even if it sees the new value of done (there is no
> synchronization). This is unlikely though.
I've confirmed this may happen on DEC Alpha. But I'm not sure this is
true for Intel x86. Would you mind providing any references for this ?
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