Messages in this thread | | | Date | Mon, 13 Dec 2021 11:09:03 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v2 3/9] atomic: Introduce atomic_{inc,dec,dec_and_test}_overflow() |
| |
On Mon, Dec 13, 2021 at 11:59:48AM +0100, Peter Zijlstra wrote: > On Mon, Dec 13, 2021 at 10:06:01AM +0000, Mark Rutland wrote: > > On Fri, Dec 10, 2021 at 05:16:21PM +0100, Peter Zijlstra wrote: > > > > +#ifndef arch_atomic_inc_overflow > > > +#define arch_atomic_inc_overflow(_v, _label) \ > > > +do { \ > > > + int __old = arch_atomic_fetch_inc(_v); \ > > > + if (unlikely(__old <= 0)) \ > > > + goto _label; \ > > > +} while (0) > > > +#endif > > > + > > > +#ifndef arch_atomic_dec_overflow > > > +#define arch_atomic_dec_overflow(_v, _label) \ > > > +do { \ > > > + int __new = arch_atomic_dec_return(_v); \ > > > + if (unlikely(__new <= 0)) \ > > > + goto _label; \ > > > +} while (0) > > > +#endif > > > + > > > +#ifndef arch_atomic_dec_and_test_overflow > > > +#define arch_atomic_dec_and_test_overflow(_v, _label) \ > > > +({ \ > > > + bool __ret = false; \ > > > + int __new = arch_atomic_dec_return(_v); \ > > > + if (unlikely(__new < 0)) \ > > > + goto _label; \ > > > + if (unlikely(__new == 0)) \ > > > + __ret = true; \ > > > + __ret; \ > > > +}) > > > +#endif > > > > I had wanted to move at least part of this to a function to ensure > > single-evaluation and avoid accidental symbol aliasing, but (as we discussed > > over IRC) I couldn't find any good way to do so, and given this is sufficiently > > specialise I think we should be ok with this as-is. It's certainly no worse > > than the existing stuff for xchg/cmpxchg. > > Right, as you know I tried the same :-) Anyway, the above macros should > be free of multi-evaluation issues, both _v and _label are only used the > once. Aliassing is always a possibility but minimized by __ prefixing > the local variables.
Agreed! I just wanted something archived in the thread mentioning that this was the only practical option.
I agree that as this stands we're not doing anything sufficiently complicated enough to worry about multi-evaluation, and that the underscores should be sufficient to avoid aliasing in practice.
Mark.
| |