Messages in this thread |  | | | Date | Fri, 3 Jul 2009 13:17:55 +0200 | | From | Ingo Molnar <> | | Subject | Re: [PATCH 1/2] FRV: Implement atomic64_t |
* Eric Dumazet <eric.dumazet@gmail.com> wrote:
> Linus Torvalds a écrit : > > > > 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. > > Using a fixed initial value (instead of __atomic64_read()) is even faster, > it apparently permits cpu to use an appropriate bus transaction. > > static inline unsigned long long atomic64_read(atomic64_t *ptr) > { > unsigned long long old = 0LL ; > > return cmpxchg8b(&ptr->counter, old, old); > }
Good point. I've done a simple:
u64 atomic64_read(atomic64_t *ptr) { return cmpxchg8b(ptr, 0, 0); } Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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