* Peter Zijlstra <[email protected]> wrote:
> On Mon, Mar 14, 2016 at 07:44:14PM -0700, Linus Torvalds wrote:
> > On Mon, Mar 14, 2016 at 5:32 AM, Ingo Molnar <[email protected]> wrote:
> > > +/**
> > > + * fetch_or - perform *ptr |= mask and return old value of *ptr
> > > + * @ptr: pointer to value
> > > + * @mask: mask to OR on the value
> > > + *
> > > + * cmpxchg based fetch_or, macro so it works for different integer types
> > > + */
> > > +#ifndef fetch_or
> > > +#define fetch_or(ptr, mask) \
> > > +({ typeof(*(ptr)) __old, __val = *(ptr); \
> > > + for (;;) { \
> > > + __old = cmpxchg((ptr), __val, __val | (mask)); \
> > > + if (__old == __val) \
> > > + break; \
> > > + __val = __old; \
> > > + } \
> > > + __old; \
> > > +})
> > > +#endif
> >
> > This is garbage.
> >
> > This macro re-uses the "mask" argument potentially many many times, so
> > semantically it's very dubious.
>
> So the below cures that; but do we want to maybe pull this back into
> sched.c and expose a version that operates on a fixed type instead?
So there are other problems with the macro - see the patch I just sent that
tries
to fix all of them.
> Although with xchg() and cmpxchg() we've already set a precedence for
> multi-width operators.
Yes, and if we name the new API 'xchg_or()' as I did it in my patch then it all
nicely fits into the existing scheme.
Thanks,
Ingo
