Re: [PATCH RESEND] random: use correct memory barriers for crng_node_pool

Eric Biggers
2021-12-20T22:10:20Z

On Mon, Dec 20, 2021 at 10:45:15PM +0100, Jason A. Donenfeld wrote:
> Hi Paul,
>
> On Mon, Dec 20, 2021 at 8:00 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > This assumes that the various crng_node_pool[i] pointers never change
> > while accessible to readers (and that some sort of synchronization applies
> > to the values in the pointed-to structure). If these pointers do change,
> > then there also needs to be a READ_ONCE(pool[nid]) in select_crng(), where
> > the value returned from this READ_ONCE() is both tested and returned.
> > (As in assign this value to a temporary.)
> >
> > But if the various crng_node_pool[i] pointers really are constant
> > while readers can access them, then the cmpxchg_release() suffices.
> > The loads from pool[nid] are then data-race free, and because they
> > are unmarked, the compiler is prohibited from hoisting them out from
> > within the "if" statement. The address dependency prohibits the
> > CPU from reordering them.
>
> Right, this is just an initialization-time allocation and assignment,
> never updated or freed again after.
>
> > So READ_ONCE() should be just fine. Which answers Jason's question. ;-)
>
> Great. So v2 of this patch can use READ_ONCE then. Thanks!

Sure, I really don't care anymore. If people want READ_ONCE() here, I'll use
it. It seems that the people who really prefer smp_load_acquire() aren't on
this thread (unlike on
https://lore.kernel.org/linux-fsdevel/20200713033330.205104-1-ebiggers@kernel.org/T/#u
for example, where READ_ONCE() was rejected), so I guess that is what people are
going to agree on in this particular case.

- Eric