Messages in this thread |  | | | Date | Wed, 1 Jul 2009 10:20:38 -0700 (PDT) | | From | Linus Torvalds <> | | Subject | Re: [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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