Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Mar 13, 2013 at 03:03:10PM +0530, anish singh wrote:
> On Tue, Mar 12, 2013 at 11:25 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Mar 12, 2013 at 04:02:47PM +0100, Frederic Weisbecker wrote:
> >> 2013/3/12 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> >> > On Tue, Mar 12, 2013 at 12:03:23PM +0800, Ming Lei wrote:
> >> >> On Tue, Mar 12, 2013 at 11:39 AM, Paul E. McKenney
> >> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> >> >
> >> >> > Atomic operations that return a value are required to act as full
> >> >> > memory
> >> >> > barriers.  This means that code relying on ordering provided by
> >> >> > these
> >> >> > atomic operations must also do ordering, either by using an explicit
> >> >> > memory barrier or by relying on guarantees from atomic operations.
> >> >> >
> >> >> > For example:
> >> >> >
> >> >> >         CPU 0                                   CPU 1
> >> >> >
> >> >> >         X = 1;                                  r1 = Z;
> >> >> >         if (atomic_inc_unless_negative(&Y)      smp_mb();
> >> >> >                 do_something();
> >> >> >         Z = 1;                                  r2 = X;
> >> >> >
> >> >> > Assuming X and Z are initially zero, if r1==1, we are guaranteed
> >> >> > that r2==1.  However, CPU 1 needs its smp_mb() in order to pair with
> >> >> > the barrier implicit in atomic_inc_unless_negative().
> >> >> >
> >> >> > Make sense?
> >> >>
> >> >> Yes, it does, and thanks for the explanation.
> >> >>
> >> >> But looks the above example is not what Frederic described:
> >> >>
> >> >> "the above atomic_read() might return -1 because there is no
> >> >> guarantee it's seeing the recent update on the remote CPU."
> >> >>
> >> >> Even I am not sure if adding one smp_mb() around atomic_read()
> >> >> can guarantee that too.
> >> >
> >> > Frederic was likely thinking of some other scenario that would be
> >> > broken by atomic_inc_unless_negative() failing to act as a full
> >> > memory barrier.  Here is another example:
> >> >
> >> >
> >> >         CPU 0                                   CPU 1
> >> >
> >> >                                                 X = 1;
> >> >         if (atomic_inc_unless_negative(&Y)      r1 = atomic_xchg(&Y,
> >> > -1);
> >> >                 r2 = X;
> >> >
> >> > If atomic_inc_unless_negative() acts as a full memory barrier, then
> >> > if CPU 0 reaches the assignment from X, the results will be guaranteed
> >> > to be 1.  Otherwise, there is no guarantee.
> >>
> >> Your scenarios show an interesting guarantee I did not think about.
> >> But my concern was on such a situation:
> >>
> >>   CPU 0                            CPU 1
> >>
> >>   atomic_set(&X, -1)
> >>                                        atomic_inc(&X)
> >>   atomic_add_unless_negative(&X, 5)
> >>
> >> On the above situation, CPU 0 may still see X == -1 and thus not add
> >> the 5. Of course all that only make sense with datas coming along.
> >
> > That could happen, but you would need CPU 1 to carry out some other
> > reference for it to be a bug.  Otherwise, CPU 1's atomic_inc() just
> 
>   CPU 0                            CPU 1
> 
>   atomic_set(&X, -1)
>                                         A =5
>                                        &X = A
>   atomic_add_unless_negative(&X, 5)
> 
> Do you mean this when you referred "carry out some other reference
> for it to be a bug"?

Not exactly.  I was thinking more of something like this, with X and
Y both initially zero:

	CPU 0					CPU 1

						Y = 1;
						smp_mb__before_atomic_dec();
	if (atomic_inc_unless_negative(&X))	atomic_dec(&X);
		r1 = 1;
	else
		r1 = Y;

If atomic_inc_unless_negative() does not imply a full memory barrier,
r1 could be equal to zero.  (Not sure where atomic_add_unless_negative()
came from, by the way, not seeing it in mainline.)

							Thanx, Paul

> > happened after all of CPU 0's code.  But yes, it would be possible
> > to misorder with some larger scenario starting with this example.
> > Especially given that atomic_inc() does not make any ordering guarantees.
> >
> >                                                         Thanx, Paul
> >
> >                                                         Thanx, Paul
> >
> > --
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