From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Eric Biggers <ebiggers@kernel.org>,
"Theodore Ts'o" <tytso@mit.edu>,
LKML <linux-kernel@vger.kernel.org>,
Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
stable <stable@vger.kernel.org>
Subject: Re: [PATCH RESEND] random: use correct memory barriers for crng_node_pool
Date: Mon, 20 Dec 2021 22:45:15 +0100 [thread overview]
Message-ID: <CAHmME9rv9RZai-0diV6kdc9yfXRog29QiStEzDpC9v25OWY81Q@mail.gmail.com> (raw)
In-Reply-To: <20211220190004.GD641268@paulmck-ThinkPad-P17-Gen-1>
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!
> 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
next prev parent reply other threads:[~2021-12-20 21:45 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-19 2:51 [PATCH RESEND] random: use correct memory barriers for crng_node_pool Eric Biggers
2021-12-20 15:07 ` Jason A. Donenfeld
2021-12-20 18:11 ` Paul E. McKenney
2021-12-20 18:16 ` Jason A. Donenfeld
2021-12-20 18:31 ` Paul E. McKenney
2021-12-20 18:35 ` Eric Biggers
2021-12-20 19:00 ` Paul E. McKenney
2021-12-20 21:45 ` Jason A. Donenfeld [this message]
2021-12-20 22:10 ` Eric Biggers
2021-12-20 15:17 ` Jason A. Donenfeld
2021-12-20 15:38 ` Eric Biggers
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