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

Eric Dumazet
2013-05-01T01:12:47Z

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...

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;
};