[vlc-devel] [PATCH] Support for Http Dynamic Streaming

Felix Abecassis felix.abecassis at gmail.com
Thu May 29 21:11:33 CEST 2014

2014-05-29 20:07 GMT+02:00 Jonathan Thambidurai <jonathan at fastly.com>:

> Hi Remi,
> Volatile (threading is irrelevant here) will stop a given (eg
> "chunk->next") from being stored in a register for future accesses.
> Thus, let us look at the following simple case (though this is not
> precisely the code),
> while(chunk->next);
> A compiler can reasonably look at the above, decide that chunk->next
> cannot change, stash it in a register and thus infinitely loop if it is not
> null.  Lets call this execution flow thread 1.
> Let us assume that a second thread t2 changes chunk->next.  Since t1 is
> storing what it believes to be chunk->next in a register (and not updating
> it from the actual memory location), t1 will get stuck in whatever state it
> was in in the first place.
> I only added the volatile after I ran into exactly the aforementioned
> phenomenon.
> Please tell me what you disagree with above.
> --Jonathan
> But accessing volatile storage does not create a memory barrier so it
might be reordered with non-volatile accesses. Also, it's not necessarily
atomic. So this is generally a bad idea.

You should use atomic variables or a synchronization primitive.

Félix Abecassis
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