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

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Message-ID: <20200921221104.GA6556@gondor.apana.org.au>
Date:   Tue, 22 Sep 2020 08:11:04 +1000
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Eric Biggers <ebiggers@...nel.org>, tytso@....edu,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        stable@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] random: use correct memory barriers for crng_node_pool

On Mon, Sep 21, 2020 at 08:27:14AM -0700, Paul E. McKenney wrote:
> On Mon, Sep 21, 2020 at 06:19:39PM +1000, Herbert Xu wrote:
> > 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?

...

> And to Eric's point, it is also true that when you have pointers to
> static data, and when the compiler can guess this, you do need something
> like smp_load_acquire().  But this is a problem only when you are (1)
> using feedback-driven compiler optimization or (2) when you compare the
> pointer to the address of the static data.

Let me restate what I think Eric is saying.  He is concerned about
the case where a->b and b is some opaque object that may in turn
dereference a global data structure unconnected to a.  The case
in question here is crng_node_pool in drivers/char/random.c which
in turn contains a spin lock.

But this reasoning could apply to any data structure that contains
a spin lock, in particular ones that are dereferenced through RCU.

So my question if this reasoning is valid, then why aren't we first
converting rcu_dereference to use smp_load_acquire?

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