Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive

[Ming Lei]

On Tue, Mar 12, 2013 at 11:39 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> 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.

Andrew, please ignore the patch, thanks.

Thanks,
-- 
Ming Lei
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/