linux-kernel - Re: Revert "gro: Fix legacy path napi

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Date:	Tue, 24 Mar 2009 23:01:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	herbert@...dor.apana.org.au, r.schwebel@...gutronix.de,
	torvalds@...ux-foundation.org, blaschka@...ux.vnet.ibm.com,
	tglx@...utronix.de, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: Revert "gro: Fix legacy path napi_complete crash",

* David Miller <davem@...emloft.net> wrote:

> From: Ingo Molnar <mingo@...e.hu>
> Date: Tue, 24 Mar 2009 21:54:44 +0100
> 
> > * Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > > Same forcedeth box i reported before. Config below. (note: if 
> > > > you want to use it you need to run it through 'make oldconfig', 
> > > > with all defaults accepted)
> > > 
> > > Hm, i just had a test failure (hung interface) with this too.
> > > 
> > > I'll go back to the original straight revert of "303c6a0: gro: Fix 
> > > legacy path napi_complete crash", and will test it overnight - to 
> > > establish a baseline of stability again. (to make sure there are 
> > > no other bugs interacting)
> > 
> > FYI, this plain revert is holding up fine in my tests so far - 50 
> > random iterations - the previous one failed after 5 iterations.
> 
> Something must be up with respect to letting interrupts in during 
> certain windows of time, or similar.
> 
> I'll take a look at this and hopefully Herbert or myself will be 
> able to figure it out.

It definitely did not show usual patterns of bug behavior - i'd have 
found it yesterday morning if it did.

I spent most of the time trying to find a reliable reproducer 
.config and system. Sometimes the bug went away with a minor change 
in the .config. Until today i didnt even suspect a mainline change 
causing this.

Also, note that i have reduced the probability of UP kernels in my 
randconfigs artificially to about 12.5% (it is 50% upstream). Still, 
despite that measure, the 'best' .config i found was an UP config - 
i dont think that's an accident. Also, i had to fully saturate the 
target CPU over gigabit to hit the bug best.

Which suggests to me (empirically) that it's indeed a race and that 
it needs a saturated system with lots of IRQs to trigger, and 
perhaps that it needs saturated/overloaded network device queues and 
complex userspace/softirq/hardirq interactions.

	Ingo
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