On 03/15, Frederic Weisbecker wrote:
>
> > 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?
>
> I feel very uncomfortable with that. The memory barrier is needed
> anyway to make sure we don't deal with a stale value of the atomic val
> (wrt. ordering against another object).
> The following should really be expected to work without added barrier:
>
> void put_object(foo *obj)
> {
> if (atomic_dec_return(obj->ref) == -1)
> free_rcu(obj);
> }
>
> bool try_get_object(foo *obj)
> {
> if (atomic_add_unless_negative(obj, 1))
> return true;
> return false;
> }
>
> = CPU 0 = = CPU 1
> rcu_read_lock()
> put_object(obj0);
> obj = rcu_derefr(obj0);
> rcu_assign_ptr(obj0, NULL);
(I guess you meant rcu_assign_ptr() then put_object())
> if (try_get_object(obj))
> do_something...
> else
> object is dying
> rcu_read_unlock()
I must have missed something.
do_something() looks fine, if atomic_add_unless_negative() succeeds
we do have a barrier?
Anyway, I understand that it is possible to write the code which
won't work without the uncoditional mb().
My point was: should we fix atomic_add_unless() then? If not, why
should atomic_add_unless_negative() differ?
Oleg.
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