Re: [PATCH net-next] af_unix: fix a fatal race with bit fields

Ben Hutchings
2013-05-01T12:08:44Z

On Wed, 2013-05-01 at 11:39 +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2013-04-30 at 18:12 -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@google.com>
> >
> > Using bit fields is dangerous on ppc64, as the compiler uses 64bit
> > instructions to manipulate them. If the 64bit word includes any
> > atomic_t or spinlock_t, we can lose critical concurrent changes.
> >
> > This is happening in af_unix, where unix_sk(sk)->gc_candidate/
> > gc_maybe_cycle/lock share the same 64bit word.
> >
> > This leads to fatal deadlock, as one/several cpus spin forever
> > on a spinlock that will never be available again.
> >
> > Reported-by: Ambrose Feinstein <ambrose@google.com>
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Paul Mackerras <paulus@samba.org>
> > ---
> >
> > Could ppc64 experts confirm using byte is safe, or should we really add
> > a 32bit hole after the spinlock ? If so, I wonder how many other places
> > need a change...
>
> Wow, nice one !
>
> I'm not even completely certain bytes are safe to be honest, though
> probably more than bitfields. I'll poke our compiler people.

There is a longstanding and hard-to-fix bug in gcc that is specific to
bitfields. I think that the underlying type isn't propagated, so when
it comes to code generation the compiler doesn't know the natural width
for the memory access.

As for bytes - early Alphas couldn't load/store less than 32 bits, but I
doubt anyone cares any more.

> The worry is of course how many more of these do we potentially have ?
> We might be able to automate finding these issues with sparse, I
> suppose.
>
> Also I'd be surprised if ppc64 is the only one with that problem... what
> about sparc64 and arm64 ?

I expect they can have the same general problem, but gcc may be more or
less keen to generate 64-bit load/store instructions for bitfields on
different architectures.

Ben.

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Ben Hutchings, Staff Engineer, Solarflare
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