Re: XDP socket rings, and LKMM litmus tests

Alan Stern
2021-03-04T03:14:52Z

On Thu, Mar 04, 2021 at 09:26:31AM +0800, Boqun Feng wrote:
> On Wed, Mar 03, 2021 at 03:22:46PM -0500, Alan Stern wrote:

> > Which brings us back to the case of the
> >
> > dep ; rfi
> >
> > dependency relation, where the accesses in the middle are plain and
> > non-racy. Should the LKMM be changed to allow this?
> >
>
> For this particular question, do we need to consider code as the follow?
>
> r1 = READ_ONCE(x); // f
> if (r == 1) {
> local_v = &y; // g
> do_something_a();
> }
> else {
> local_v = &y;
> do_something_b();
> }
>
> r2 = READ_ONCE(*local_v); // e
>
> , do we have the guarantee that the first READ_ONCE() happens before the
> second one? Can compiler optimize the code as:
>
> r2 = READ_ONCE(y);
> r1 = READ_ONCE(x);

Well, it can't do that because the compiler isn't allowed to reorder
volatile accesses (which includes READ_ONCE). But the compiler could
do:

r1 = READ_ONCE(x);
r2 = READ_ONCE(y);

> if (r == 1) {
> do_something_a();
> }
> else {
> do_something_b();
> }
>
> ? Although we have:
>
> f ->dep g ->rfi ->addr e

This would be an example of a problem Paul has described on several
occasions, where both arms of an "if" statement store the same value
(in this case to local_v). This problem arises even when local
variables are not involved. For example:

if (READ_ONCE(x) == 0) {
WRITE_ONCE(y, 1);
do_a();
} else {
WRITE_ONCE(y, 1);
do_b();
}

The compiler can change this to:

r = READ_ONCE(x);
WRITE_ONCE(y, 1);
if (r == 0)
do_a();
else
do_b();

thus allowing the marked accesses to be reordered by the CPU and
breaking the apparent control dependency.

So the answer to your question is: No, we don't have this guarantee,
but the reason is because of doing the same store in both arms, not
because of the use of local variables.

Alan