on den 18.05.2005 Klokka 12:53 (+0200) skreiv Miklos Szeredi:
> > Yes you can. cmpxchg() is atomic. Several archs implement atomic_inc() and co
> > with cmpxchg() or similar.
> >
> > Something like:
> >
> > static inline struct namespace *grab_namespace(struct namespace *n)
> > {
> > int old = atomic_read(&n->count);
> >
> > while (old > 0) {
> > /* attempt to increment the counter */
> > old = cmpxchg(&n->count, old, old + 1);
> > }
> >
> > return old > 0 ? n : NULL;
> > }
> >
>
> Ahh OK :)
>
> There's still the problem of cmpxchg meddling in the internals of an
> atomic_t. Is that OK? Will that work on all archs?
Some archs already have an atomic_dec_if_positive() (see for instance
the PPC). It won't take much work to convert that to an
atomic_inc_if_positive().
For those arches that don't have that sort of thing, then writing a
generic atomic_inc_if_positive() using cmpxchg() will often be possible,
but there are exceptions (for instance the original 386 does not have a
cmpxchg, so there you will have to use something else).
Cheers,
Trond
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