[linux-audio-dev] crossplatform atomics

[Kai Vehmanen]

On Wed, 24 May 2006, Paul Davis wrote:

>>> sizeof(int) is your friend
> not sure what you mean, but on sparcs, int writes are not atomic unless
> you only use the lower 24 bits.

Does someone have a good reference on this? I think the writes just are 
not atomic, but you can use some tricks [1] to implement atomic behaviour 
by spinning until the operation succeeds.

[1] http://www.freepatentsonline.com/5666546.html?highlight=5434995
     .. I'm not sure maybe this is just one variation...

But, reading the Linux 2.6.16.17 code for sparc (atomic.h and atomic.S), 
only the writes are protected, not reads. So as long as you have only one 
writer, you should be safe. Also, the 24bit limitation only applies if you 
are using the low 8bits for spinning.

Anyways, a good (accurate and available online) reference for this would 
be nice to have, to be 100% sure before making any architectural 
decisions. For me, I'd be happy to just exclude these platforms, or use 
mutexes as a crude workaround on the affected platforms (assuming there're 
only few of them).

Cross-platform atomic integer operations would be nice, but you can do 
quite a lot with just atomic read/writes, which most of the platforms 
already provide for 'int's. For example, these are sufficient to implement 
a simple one-writer-one-reader lock-free queue (which is necessary in most 
audio apps).

OTOH, it is probably wise to explicitly mark all code where ints are 
assumed to be atomic w.r.t. read/writes (or any other non-standard 
properties). Especially if sparc-style atomic properties become more 
widely used later on...

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