From: Herbert Xu <herbert@gondor.apana.org.au>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Eric Biggers <ebiggers@kernel.org>,
tytso@mit.edu, linux-kernel@vger.kernel.org,
linux-crypto@vger.kernel.org, stable@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] random: use correct memory barriers for crng_node_pool
Date: Tue, 22 Sep 2020 08:11:04 +1000 [thread overview]
Message-ID: <20200921221104.GA6556@gondor.apana.org.au> (raw)
In-Reply-To: <20200921152714.GC29330@paulmck-ThinkPad-P72>
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@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
next prev parent reply other threads:[~2020-09-21 22:11 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-16 23:30 [PATCH] random: use correct memory barriers for crng_node_pool Eric Biggers
2020-09-17 7:26 ` Herbert Xu
2020-09-17 16:58 ` Eric Biggers
2020-09-21 8:19 ` Herbert Xu
2020-09-21 15:27 ` Paul E. McKenney
2020-09-21 22:11 ` Herbert Xu [this message]
2020-09-21 23:26 ` Paul E. McKenney
2020-09-21 23:51 ` Herbert Xu
2020-09-22 18:42 ` Paul E. McKenney
2020-09-22 18:59 ` Eric Biggers
2020-09-22 20:31 ` Paul E. McKenney
2020-09-21 23:52 ` Eric Biggers
2020-09-22 18:31 ` Paul E. McKenney
2020-09-22 19:09 ` Eric Biggers
2020-09-22 20:56 ` Paul E. McKenney
2020-09-22 21:55 ` Eric Biggers
2020-09-25 0:59 ` Paul E. McKenney
2020-09-25 2:09 ` Eric Biggers
2020-09-25 3:31 ` Paul E. McKenney
2020-10-02 3:07 ` Eric Biggers
2020-10-08 18:31 ` Paul E. McKenney
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