linux-kernel - Re: [PATCH RESEND] random: use correct memory barriers for crng_node

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Date:   Mon, 20 Dec 2021 16:10:11 -0600
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH RESEND] random: use correct memory barriers for
 crng_node_pool

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@...nel.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