netdev - Re: kernel panic in fib_rules_lookup [2.6.27.7 vendor-patched]

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Date:	Tue, 26 Oct 2010 09:29:48 -0400
From:	Joe Buehler <aspam@....net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org
Subject: Re: kernel panic in fib_rules_lookup [2.6.27.7 vendor-patched]

Eric Dumazet wrote:

> With a normal workload, on a dual cpu machine, a missing memory barrier
> can stay un-noticed for quite a long time. The race window is so small
> that probability for the bug might be 0.0000001 % or something like
> that :(

I'm looking at the LINUX source at the moment and not liking what I see
in include/asm-mips/barrier.h:

#define smp_mb()	__asm__ __volatile__(__WEAK_ORDERING_MB : : :"memory")
#ifdef CONFIG_CPU_CAVIUM_OCTEON
#define smp_rmb()	barrier()
#define smp_wmb()	barrier()
#else
#define smp_rmb()	__asm__ __volatile__(__WEAK_ORDERING_MB : : :"memory")
#define smp_wmb()	__asm__ __volatile__(__WEAK_ORDERING_MB : : :"memory")
#endif

The Octeon documentation explicitly says that neither loads nor stores
need execute in program order, so the definitions for smp_rmb and
smp_wmb appear to be wrong wrong wrong.

It appears that smp_wmb should be making use of SYNCW and smp_rmb should
be making use of SYNC.

Should I pursue this question on the main LINUX kernel list?

Joe Buehler
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