Re: [PATCH v2 net-next] af_unix: fix a fatal race with bit fields

David Miller
2013-05-01T19:14:21Z

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 01 May 2013 08:24:03 -0700

> [PATCH v2] af_unix: fix a fatal race with bit fields
>
> Using bit fields is dangerous on ppc64/sparc64, as the compiler [1]
> 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.
>
> A safer way would be to use a long to store flags.
> This way we are sure compiler/arch wont do bad things.
>
> As we own unix_gc_lock spinlock when clearing or setting bits,
> we can use the non atomic __set_bit()/__clear_bit().
>
> recursion_level can share the same 64bit location with the spinlock,
> as it is set only with this spinlock held.
>
> [1] bug fixed in gcc-4.8.0 :
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52080
>
> Reported-by: Ambrose Feinstein <ambrose@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied and queued up for -stable, thanks Eric.