From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
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 10:11:15 -0800 [thread overview]
Message-ID: <20211220181115.GZ641268@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <CAHmME9pQ4vp0jHpOyQXHRbJ-xQKYapQUsWPrLouK=dMO56y1zA@mail.gmail.com>
On Mon, Dec 20, 2021 at 04:07:28PM +0100, Jason A. Donenfeld wrote:
> Hi Eric,
>
> This patch seems fine to me, and I'll apply it in a few days after
> sitting on the list for comments, but:
>
> > Note: READ_ONCE() could be used instead of smp_load_acquire(), but it is
> > harder to verify that it is correct, so I'd prefer not to use it here.
> > (https://lore.kernel.org/lkml/20200916233042.51634-1-ebiggers@kernel.org/T/#u),
> > and though it's a correct fix, it was derailed by a debate about whether
> > it's safe to use READ_ONCE() instead of smp_load_acquire() or not.
>
> But holy smokes... I chuckled at your, "please explain in English." :)
>
> Paul - if you'd like to look at this patch and confirm that this
> specific patch and usage is fine to be changed into READ_ONCE()
> instead of smp_load_acquire(), please pipe up here. And I really do
> mean this specific patch and usage, not to be confused with any other
> usage elsewhere in the kernel or question about general things, which
> doubtlessly involve larger discussions like the one Eric linked to
> above. If you're certain this patch here is READ_ONCE()able, I'd
> appreciate your saying so with a simple, "it is safe; go for it",
> since I'd definitely like the optimization if it's safe. If I don't
> hear from you, I'll apply this as-is from Eric, as I'd rather be safe
> than sorry.
First I would want to see some evidence that READ_ONCE() was really
providing measurable performance benefit. Such evidence would be
easiest to obtain by running on a weakly ordered system such as ARM,
ARMv8, or PowerPC.
If this does provide a measurable benefit, why not the following?
static inline struct crng_state *select_crng(void)
{
struct crng_state **pool;
struct crng_state *pooln;
int nid = numa_node_id();
/* pairs with cmpxchg_release() in do_numa_crng_init() */
pool = rcu_dereference(&crng_node_pool);
if (pool) {
pooln = rcu_dereference(pool[nid]);
if (pooln)
return pooln;
}
return &primary_crng;
}
This is in ignorance of the kfree() side of this code. So another
question is "Suppose that there was a long delay (vCPU preemption, for
example) just before the 'return pooln'. What prevents a use-after-free
bug?"
Of course, this question applies equally to the smp_load_acquire()
version.
Thanx, Paul
next prev parent reply other threads:[~2021-12-20 18:11 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 [this message]
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
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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