linux-kernel - Re: Process Hang in __read_seqcount

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Date:	Fri, 26 Oct 2012 22:53:04 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter LaDow <petela@...ougs.wsu.edu>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	netfilter@...r.kernel.org
Subject: Re: Process Hang in __read_seqcount_begin

On Fri, 26 Oct 2012, Peter LaDow wrote:
> (I've added netfilter and linux-rt-users to try to pull in more help).
> 
> On Fri, Oct 26, 2012 at 9:48 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Upstream kernel is fine, there is no race, as long as :
> >
> > local_bh_disable() disables BH and preemption.
> 
> Looking at the unpatched code in net/ipv4/netfilter/ip_tables.c, it
> doesn't appear that any of the code checks the return value for
> xt_write_receq_begin to determine if it is safe to write.  And neither
> does the newly patched code.  How did the mainline code prevent
> corruption of the tables it is updating?
> 
> Why isn't there something like:
> 
>   while ( (addend = xt_write_recseq_begin()) == 0 );
> 
> To make sure that only one person has write access to the tables?
> Better yet, why not use a seqlock_t instead?
> 
> > Apparently RT changes this, so RT needs to change the code.
> 
> The RT patch only touches local_bh_disable/enable, not the code in
> ip_tables.c.  Does the local_bh_disable/enable in the mainline code
> protect against multiple writers?
> 
> > cmpxchg() has strong guarantees (and is also slower than upstream code),
> > and seems a reasonable way to fix RT/iptables
> 
> I see this now.  And I agree that your patch would prevent corruption
> of the sequence counter.

Thanks for the reminder. I'll have a look.

       tglx
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