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DateWed, 1 Jul 2009 10:20:38 -0700 (PDT)
FromLinus Torvalds <>
SubjectRe: [PATCH 1/2] FRV: Implement atomic64_t


On Wed, 1 Jul 2009, David Howells wrote:
> +
> +#define ATOMIC64_INIT(i) { (i) }
> +#define atomic64_read(v) ((v)->counter)
> +#define atomic64_set(v, i) (((v)->counter) = (i))

These seem to be buggy.

At least "atomic64_read()" needs to make sure to actually read it
atomically - otherwise you'll do two 32-bit reads, and that just gets
crap. Imagine if somebody is adding 1 to 0x00000000ffffffff, and then
"atomic64_read()" reads it as two accesses in the wrong place, and gets
either 0, or 0x00000001ffffffff, both of which are totally incorrect.
The case of 'atomic64_set()' is _slightly_ less clear, because I think we
use it mostly for initializers, so atomicity is often not strictly
required. But at least on x86, we do guarantee that it sets it atomically
too.

Btw, Ingo: I looked at the x86-32 versions to be sure, and noticed a
couple of buglets:

- atomic64_xchg uses "atomic_read()". Sure, it happens to work, since
the "atomic_read()" is not type-safe, and gets a non-atomic 64-bit
read, but that looks really really bogus.

It _should_ use __atomic64_read(), and the 64-bit versions should use a
different counter name ("counter64"?) or we should use an inline
function for atomic_read(), so that the type safety issue gets fixed.

- atomic64_read() is being stupid with the whole loop thing. It _should_
just do

static inline unsigned long long atomic64_read(atomic64_t *ptr)
{
unsigned long long old = __atomic64_read(ptr);
return cmpxchg8b(ptr, old, old);
}
and that's it. No loop. cmpxchg8b() will return the right thing.

- Similarly, atomic64_add_return() is bogus for the same reasons: using
the wrong 'atomic_read()', and unnecessarily ignoring the returned old
value. It probably should do

static inline unsigned long long
atomic64_add_return(unsigned long long delta, atomic64_t *ptr)
{
unsigned long long old;
old = __atomic_read64(ptr);
for (;;) {
unsigned long long tmp, new;
new = old + delta;
tmp = atomic64_cmpxchg(ptr, old, new);
if (tmp == old)
return new;
old = tmp;
}
}
or something. NOTE NOTE NOTE! Not tested!

Those functions also almost certainly should _not_ be inlined. They need
so many registers that inlining them is crazy.

Linus


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