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

[Jason A. Donenfeld]

Hi Paul,

On Mon, Dec 20, 2021 at 8:00 PM Paul E. McKenney <paulmck@xxxxxxxxxx> 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!

> Looking at _extract_crng(), if this was my code, I would use READ_ONCE()
> in the checks, but that might be my misunderstanding boot-time constraints
> or some such.  Without some sort of constraint, I don't see how the code
> avoids confusion from reloads of crng->init_time if two CPUs concurrently
> see the expiration of CRNG_RESEED_INTERVAL, but I could easily be missing
> something that makes this safe.  (And this is irrelevant to this patch.)

Indeed init_time seems to race via the crng_reseed path, and
READ_ONCE()ing that seems reasonable. The other setters of it --
initialize_{primary,secondary} -- are in the boot path.

Jason