On 03/15, Ming Lei wrote:
>
> On Fri, Mar 15, 2013 at 9:46 PM, Oleg Nesterov <o...@redhat.com> wrote:
> > On 03/15, Ming Lei wrote:
> >>
> >> On Fri, Mar 15, 2013 at 12:24 AM, Oleg Nesterov <o...@redhat.com> wrote:
> >> > static inline int atomic_inc_unless_negative(atomic_t *p)
> >> > {
> >> > int v, v1;
> >> > - for (v = 0; v >= 0; v = v1) {
> >> > + for (v = atomic_read(p); v >= 0; v = v1) {
> >> > v1 = atomic_cmpxchg(p, v, v + 1);
> >>
> >> Unfortunately, the above will exchange the current value even though
> >> it is negative, so it isn't correct.
> >
> > Hmm, why? We always check "v >= 0" before we try to do
> > atomic_cmpxchg(old => v) ?
>
> Sorry, yes, you are right. But then your patch is basically same with the
> previous one, isn't it?
Sure, the logic is the same, just the patch (and the code) looks simpler
and more understandable.
> And has same problem, see below discussion:
>
> http://marc.info/?t=136284366900001&r=1&w=2
The lack of the barrier?
I thought about this, this should be fine? atomic_add_unless() has the same
"problem", but this is documented in atomic_ops.txt:
atomic_add_unless requires explicit memory barriers around the operation
unless it fails (returns 0).
I thought that atomic_add_unless_negative() should have the same
guarantees?
Paul? Frederic?
Oleg.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
