Re: Broken ARM atomic ops wrt memory barriers (was : [PATCH] Addcmpxchg support for ARMv6+ systems)

From: Russell King - ARM Linux
Date: Mon May 25 2009 - 15:34:51 EST

On Mon, May 25, 2009 at 01:29:55PM -0400, Mathieu Desnoyers wrote:
> Basically, to make sure we don't forget anything, someone should go
> through all the atomic_ops.txt document once more and audit all ARM
> primitives.

That's easy to say - it took me more than half an hour of reading through
atomic_ops.txt to work out what was required for things like cmpxchg,
xchg, etc because it's _very_ waffley and verbose, and directs you to
other parts of the document.

For example, for atomic_cmpxchg it directs you to 'cas' but cas doesn't
really say what the barrier requirements are - reading it leaves me
to expect that provided spinlocks are serializing themselves, it's
fine if 'cas' itself isn't.

Maybe if someone has a few days to translate atomic_ops.txt into a
succinct set of requirements, and get _that_ reviewed, then we could
properly audit this stuff.
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