[Jack-Devel] Volatile vs memory fences

[Sean Bolton]

Hi Xavier,

On Sat Dec  5 2015, Xavier Mendez wrote:
> I was reviewing the code for `jack_ringbuffer` and I saw it uses 
> `volatile` for the pointers. This confused me, since my teachers have 
> long insisted that `volatile` isn't for use on multithreaded code.
> 
> Take i.e. `jack_ringbuffer_write`: the hardware could reorder the
> writes so that the pointer is updated before the buffer has been
> written. Wouldn't a memory fence be required?

You're quite right--memory barriers are required for safe operation of
the ringbuffer on systems with weak memory ordering. IIRC, this is
at least the third time this has come up on the list (see [1] for one
previous discussion), but it's never been fixed. The number of people
running jack on affected hardware is very small, and it is difficult to
convince people that the current code could fail on those systems. Take
a look at the PortAudio ringbuffer if you'd like to see it done right.

-Sean

[1] jack-devel mailing list, 22 Oct 2008, "Re: latest svn commit: 64/32
support". http://comments.gmane.org/gmane.comp.audio.jackit/17887