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

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Message-ID: <20200922185931.GA1616407@gmail.com>
Date:   Tue, 22 Sep 2020 11:59:31 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>, 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 Tue, Sep 22, 2020 at 11:42:43AM -0700, Paul E. McKenney wrote:
> On Tue, Sep 22, 2020 at 09:51:36AM +1000, Herbert Xu wrote:
> > On Mon, Sep 21, 2020 at 04:26:39PM -0700, Paul E. McKenney wrote:
> > >
> > > > But this reasoning could apply to any data structure that contains
> > > > a spin lock, in particular ones that are dereferenced through RCU.
> > > 
> > > I lost you on this one.  What is special about a spin lock?
> > 
> > I don't know, that was Eric's concern.  He is inferring that
> > spin locks through lockdep debugging may trigger dependencies
> > that require smp_load_acquire.
> > 
> > Anyway, my point is if it applies to crng_node_pool then it
> > would equally apply to RCU in general.
> 
> Referring to the patch you call out below...
> 
> Huh.  The old cmpxchg() primitive is fully ordered, so the old mb()
> preceding it must have been for correctly interacting with hardware on
> !SMP systems.  If that is the case, then the use of cmpxchg_release()
> is incorrect.  This is not the purview of the memory model, but rather
> of device-driver semantics.  Or does crng not (or no longer, as the case
> might be) interact with hardware RNGs?

No hardware involved here.  The mb() is just unnecessary, as I noted in my patch
https://lore.kernel.org/lkml/20200916233042.51634-1-ebiggers@kernel.org/.

> What prevents either the old or the new code from kfree()ing the old
> state out from under another CPU that just now picked up a pointer to the
> old state?  The combination of cmpxchg_release() and smp_load_acquire()
> won't do anything to prevent this from happening.  This is after all not
> a memory-ordering issue, but instead an object-lifetime issue.  But maybe
> you have a lock or something that provides the needed protection.  I don't
> see how this can be the case and still require the cmpxchg_release()
> and smp_load_acquire(), but perhaps this is a failure of imagination on
> my part.

crng_node_pool is initialized only once, and never freed.

- Eric