Re: [LAD] Atomic Operations

>>>>> this was discussed at some considerable length on jack-devel last  
>>>>> year, IIRC.
>>>>> for single reader/single writer ringbuffers, i believe that we
>>>>> concluded that memory barriers are not necessary.
>>>> No, to me the conclusion was: we can't programmatically prove that  
>>>> memory
>>>> barriers are needed (even on the most vulnerable architectures),  
>>>> but the theory
>>>> say that they are, and they should be added for correctness.
>>> My understanding matches Olivier's.  Intel processors have strong memory
>>> ordering, and so on them the jack ringbuffer is safe without memory
>>> barriers. However, some PPC processors, and SPARC V9s under linux  
>>> (but not
>>> Solaris), use weak memory ordering, and on them, the jack ringbuffer  
>>> code
>>> can theoretically fail.
>>
>> exactly, the issue may not appear on x86, because of its memory
>> consistency, weakly-ordered machines will need some barriers ...
> 
> The point is, I wrote ringbuffer stress tests, and someone reported on this list
> that they succeeded on a weakly-ordered (PowerPC SMP) system, even without
> memory barriers. Anyway...

unfortunately stress tests won't necessarily show the race condition


>>> See the "ring buffer memory barriers" discussion on jack-devel back in
>>> October of last year for more information; in particular, this article
>>> by Paul E. McKenney is very helpful:
>>>
>>> http://www.linuxjournal.com/article/8211
>>
>> memory-barriers.txt of the linux kernel documentation is interesting as
>> well ...
> 
> The portaudio ringbuffer is also a good source of inspiration I think:
> http://portaudio.com/trac/browser/portaudio/trunk/src/common/pa_ringbuffer.c
> 
> In short, they use memory barriers at only three places, especially:
> 
> - "to ensure that previous writes are seen before we update the write index"
> - "to ensure that previous writes are always seen before updating the (read) index."

the kfifo doesn't look too different, either ...

tim

[1]
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob_plain;f=kernel/kfifo.c;hb=HEAD

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