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

Anton Blanchard
2013-05-01T01:51:07Z

Hi Eric,

> 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.

I just spoke to Alan Modra and he suspects this is a compiler
bug. Can you give us your compiler version info?

Anton

> 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...
>
> include/net/af_unix.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> index a8836e8..4520a23f 100644
> --- a/include/net/af_unix.h
> +++ b/include/net/af_unix.h
> @@ -57,8 +57,8 @@ struct unix_sock {
> struct list_head link;
> atomic_long_t inflight;
> spinlock_t lock;
> - unsigned int gc_candidate : 1;
> - unsigned int gc_maybe_cycle : 1;
> + unsigned char gc_candidate;
> + unsigned char gc_maybe_cycle;
> unsigned char recursion_level;
> struct socket_wq peer_wq;
> };
>
>
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