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

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Date:	Tue, 30 Apr 2013 18:12:41 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ambrose Feinstein <ambrose@...gle.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH net-next] af_unix: fix a fatal race with bit fields

From: Eric Dumazet <edumazet@...gle.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@...gle.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Paul Mackerras <paulus@...ba.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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