Re: +atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive .patchadded to -mm tree

[Paul E. McKenney]

On Fri, Mar 15, 2013 at 07:34:32PM +0100, Frederic Weisbecker wrote:
> 2013/3/15 Oleg Nesterov <oleg@xxxxxxxxxx>:
> > 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())
> 
> Right.
> 
> >
> >>                                 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?
> 
> Ok, I guess the guarantee of a barrier in case of failure is probably
> not needed. But since the only way to safely read the atomic value is
> a cmpxchg like operation, I guess a barrier must be involved in any
> case.
> 
> Using atomic_read() may return some stale value.
> 
> >
> > Anyway, I understand that it is possible to write the code which
> > won't work without the uncoditional mb().
> 
> Yeah that's my fear.
> 
> >
> > My point was: should we fix atomic_add_unless() then? If not, why
> > should atomic_add_unless_negative() differ?
> 
> They shouldn't differ I guess.

Completely agreed.  It is not like memory ordering is simple, so we should
keep the rules simple.  Atomic primitives that sometimes imply a memory
barrier seems a bit over the top.

The rule is that if an atomic primitive returns non-void, then there is
a full memory barrier before and after.  This applies to primitives
returning boolean as well, with atomic_dec_and_test() setting this
precedent from what I can see.

							Thanx, Paul

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