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

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Message-Id: <20130501.151419.1237664751661506825.davem@davemloft.net>
Date:	Wed, 01 May 2013 15:14:19 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	benh@...nel.crashing.org, netdev@...r.kernel.org, paulus@...ba.org,
	ambrose@...gle.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 net-next] af_unix: fix a fatal race with bit fields

From: Eric Dumazet <eric.dumazet@...il.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@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied and queued up for -stable, thanks Eric.
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