Re: [RFC][PATCH 0/5] arch: atomic rework

[Torvald Riegel <triegel@redhat.com>]

On Thu, 2014-02-27 at 11:47 -0800, Linus Torvalds wrote:
> On Thu, Feb 27, 2014 at 11:06 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> >
> > 3.      The comparison was against another RCU-protected pointer,
> >         where that other pointer was properly fetched using one
> >         of the RCU primitives.  Here it doesn't matter which pointer
> >         you use.  At least as long as the rcu_assign_pointer() for
> >         that other pointer happened after the last update to the
> >         pointed-to structure.
> >
> > I am a bit nervous about #3.  Any thoughts on it?
> 
> I think that it might be worth pointing out as an example, and saying
> that code like
> 
>    p = atomic_read(consume);
>    X;
>    q = atomic_read(consume);
>    Y;
>    if (p == q)
>         data = p->val;
> 
> then the access of "p->val" is constrained to be data-dependent on
> *either* p or q, but you can't really tell which, since the compiler
> can decide that the values are interchangeable.

The wording I proposed would make the p dereference have a value
dependency unless X and Y would somehow restrict p and q.  The reasoning
is that if the atomic loads return potentially more than one value, then
even if we find out that two such loads did return the same value, we
still don't know what the exact value was.