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

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Message-ID: <1287805487.2658.5.camel@edumazet-laptop>
Date:	Sat, 23 Oct 2010 05:44:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Buehler <aspam@....net>
Cc:	netdev@...r.kernel.org
Subject: Re: kernel panic in fib_rules_lookup [2.6.27.7 vendor-patched]

Le vendredi 22 octobre 2010 à 16:30 -0400, Joe Buehler a écrit :
> Eric Dumazet wrote:
> 
> > Could you provide a disassembly of function fib_rules_lookup ?
> 
> Try looking in http://68.100.141.95:3000/linux-crash/.  There should be
> the source file I am using (not current release if you recall), the .o,
> the disassembly, and a -S compile that makes deducing the line numbers a
> little easier.
> 

Hmm, I'll take a look sometime in the future, thanks

> I have tried adding a recursive spinlock as a quick fix without much
> success.  It looks like the code in net/core/fib_rules.c results in some
> rather complex code paths through the kernel involving softirq handlers.
>  I get lockups though I took steps to make sure local interrupts were
> off and preemption disabled when taking the spinlock.
> 
> I do not know the RCU API or the network code at all but some of the
> code in fib_rules.c does not look right prima facie.  For example,
> dump_rules does not seem to bother about RCU when traversing rules_list.
>  Its caller, fib_nl_dumprule does, but only for one of two calls.  The
> fib_nl_newrule and fib_nl_delrule functions traverse rules_list without
> any obvious RCU precautions.

There is no obligation RCU shall be used in a slow path algorithm.

This slow path uses regular locking to prevent another process to change
the things while we read them, thats it.

Patrick made a change in the past so that RTNL is locked on dump
operations, so no dump_rules() is in fact protected by RTNL, it doesnt
need to take care of 'RCU'.

Thanks



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