netdev - Re: + ppp_generic-fix-lockdep-warning.patch added to -mm tree

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Date:	Tue, 17 Apr 2007 08:26:32 -0500
From:	Michal Ostrowski <mostrows@...thlink.net>
To:	Jarek Poplawski <jarkao2@...pl>
CC:	"Yuriy N. Shkandybin" <jura@...ams.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>, netdev@...r.kernel.org,
	Michal Ostrowski <mostrows@...akeasy.net>
Subject: Re: + ppp_generic-fix-lockdep-warning.patch added to -mm tree

The "xmit" function of a PPP channel is a synchronous operation.  If the 
transmission fails, we must notify the caller and let them re-submit the 
skb later.  The return status of dev_queue_xmit is needed to determine 
the return code passed back to the caller and thus the call is made 
synchronously and not in a tasklet.

Looking at the stack traces earlier in this thread, it seems to me that 
even if the PPPoE call was made in a tasklet, this same warning could be 
generated.

--
Michal Ostrowski
mostrows@...thlink.net



Jarek Poplawski wrote:
> On Wed, Apr 11, 2007 at 12:52:28PM +0400, Yuriy N. Shkandybin wrote:
> ...
>>> On Wed, 11 Apr 2007 09:57:33 +0400 "Yuriy N. Shkandybin" <jura@...ams.com> 
>>> wrote:
>>>
>>>> I've tested  2.6.21-rc6-mm1
>>>> Linux vpn1 2.6.21-rc6-mm1 #4 SMP Wed Apr 11 03:34:26 MSD 2007 x86_64
>>>> Intel(R) Pentium(R) D CPU 2.80GHz GenuineIntel GNU/Linux
>>>>
>>>> warn appeares upon first pppoe connection to rp-pppoe server in kernel 
>>>> mode
>>>>
>>> Thanks.  So you're saying that
>>> ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.21-rc6/2.6.21-rc6-mm1/broken-out/ppp_generic-fix-lockdep-warning.patch
>>> did not fix anything?
>> As i understand this patch already in -mm tree, so I've booted into last mm 
>> kernel and received this locked warning.
>> Or i've mistaked and should apply this patch manually?
> 
> Hi!
> 
> Yuriy - thanks for testing my patch ...(pause) Not!
> 
> It seems this patch is not visible in this version - probably
> is overpatched by something else. But your new log shows there
> is another connection between these locks (ppp_xmit_process
> and ppp_push instead of ppp_channel_push in "-> #0"), so the
> patch is not sufficient (and could be dumped).
> 
> I don't know your vlans configuration, but it seems the real
> lockup isn't very probable here - it's rather lockdep question.
> I think vlan's too broad lockdep class is the main "guilty"
> here, but probably pppoe also could be enhanced: it's making
> the things unnecessarily complicated by calling dev_queue_xmit
> under ppp_generic's xmit locks. I wonder if there is any reason
> against using a tasklet here.
> 
> I'll try to find more time to untie this yet - or maybe some
> maintainer will find this interesting, too...
> 
> Regards,
> Jarek P.
> 
> PS: sorry for late responding (vacations).
> 
> 
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