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[Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravi

To: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Subject: [Xen-devel] Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable
From: Andi Kleen <ak@xxxxxxx>
Date: Tue, 20 Mar 2007 18:42:00 +0100
Cc: jeremy@xxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, mingo@xxxxxxx, Andi Kleen <ak@xxxxxxx>, jbeulich@xxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, chrisw@xxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, anthony@xxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, torvalds@xxxxxxxxxxxxxxxxxxxx, David Miller <davem@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
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On Tue, Mar 20, 2007 at 10:25:20AM -0600, Eric W. Biederman wrote:
> What I recall observing is call traces that made no sense.  Not just
> extra noise in the stack trace but things like seeing a function that
> has exactly one path to it, and not seeing all of the functions on
> that path in the call trace.

That's tail call/sibling call optimization. No unwinder can untangle that
because the return address is lost.  But it's also an quite important 

> >
> > In 2.4 it was often very reasonable to just sort out the false positives,
> > but with sometimes 20-30+ level deep call chains in 2.6 with many callbacks 
> > that
> > just
> > gets far too tenuous. 
> Hmm.  I haven't seen those traces, but I wonder if the size of those
> stack traces indicates potential stack overflow problems.

Most functions have quite small frames, so 20-30 is still not a problem

> Do you also validate the unwind data?

There are many sanity checks in the unwind code and it will fall back
to the old unwinder when it gets stuck.

> > Although in future it would be good if people did some more analysis in root
> > causes for failures before let the paranoia take over and revert patches.
> >
> > We see a good example here of what I call the JFS/ACPI effect: code gets 
> > merged
> > too early with some visible problems. It gets a bad name and afterwards 
> > people
> > never look objectively at it again and just trust their prejudices. 
> I don't know.  The impression I got was the root cause analysis stopped 
> when it was observed that the code was unsuitable for solving the problem.

No, me and Jan fixed all reported bugs as far as I know.


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