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

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Message-ID: <20200921081939.GA4193@gondor.apana.org.au>
Date:   Mon, 21 Sep 2020 18:19:39 +1000
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     tytso@....edu, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, stable@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] random: use correct memory barriers for crng_node_pool

On Thu, Sep 17, 2020 at 09:58:02AM -0700, Eric Biggers wrote:
>
> smp_load_acquire() is obviously correct, whereas READ_ONCE() is an optimization
> that is difficult to tell whether it's correct or not.  For trivial data
> structures it's "easy" to tell.  But whenever there is a->b where b is an
> internal implementation detail of another kernel subsystem, the use of which
> could involve accesses to global or static data (for example, spin_lock()
> accessing lockdep stuff), a control dependency can slip in.

If we're going to follow this line of reasoning, surely you should
be converting the RCU derference first and foremost, no?

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt