Linus Torvalds wrote:
On Thu, 7 Dec 2006, Roman Zippel wrote:
On Wed, 6 Dec 2006, Matthew Wilcox wrote:
To be honest, it'd be much easier if we only defined these operations on
atomic_t's. We have all the infrastructure in place for them, and
they're fairly well understood. If you need different sizes, I'm OK
with an atomic_pointer_t, or whatever.
FWIW Seconded.
I disagree.
Any _real_ CPU will simply never care about _anything_ else than just the
size of the datum in question. There's absolutely no point to only allow
it on certain types, especially as we _know_ those certain types are
already going to be more than one, and we also know that they are going to
be different sizes. In other words, in reality, we have to handle a
sizeable subset of the whole generic situation, and the "on certain types
only" situation is only going to be awkward and irritating.
For example, would we have a different "cmpxchg_ptr()" function for the
atomic pointer thing? With any reasonable CPU just depending on the _size_
of the type, I don't see what the problem is with just doing the
bog-standard "cmpxchg_8/16/32/64" and having the bog-standard case-
statement in a header file to do it all automatically for you, and then we
don't need to worry about it.
What's wrong with using atomic_cmpxchg? For unsigned long / pointers,
there is a patch to implement atomic_long_cmpxchg.
Some architectures simply can't implement cmpxchg on memory that may
be modified in arbitrary ways. If you add some more conditions to use
cmpxchg, then you weaken it (ie. can't use it for synchronisation
with userspace) and on top of that you don't get the easy static
checking that atomic_t gives you.
--
SUSE Labs, Novell Inc.
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