Re: [PATCH] fix race in mark_mounts_for_expiry()

[Trond Myklebust <trond.myklebust@fys.uio.no>]

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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