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SubjectRe: [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.


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