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

To: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
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 20:21:19 +0100
Cc: jeremy@xxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, mingo@xxxxxxx, Andi Kleen <ak@xxxxxxx>, jbeulich@xxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, chrisw@xxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>, anthony@xxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, David Miller <davem@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx
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On Tue, Mar 20, 2007 at 10:27:00AM -0700, Linus Torvalds wrote:
> On Tue, 20 Mar 2007, Andi Kleen wrote:
> > 
> > The code never did that. In fact many of the problems we had initially
> > especially came out of that -- the fallback code that would handle
> > this case wasn't fully correct.
> I don't keep my emails any more, but you *never* fixed the problems in 
> arch/*/kernel/traps.c.

I fixed that one after you dropped it (hmm, double checking: or at least
I thought I had fixed it, but don't see the code right now; will redo then) 
Basically it was just a one liner anyways - always check against all the
stacks that are there.

> Yes, the kernel/unwind.c issues generally got fixed. The infinite loops in 
> the *callers* never did.

There was later a weaker form that should have caught most loops, but admittedly
it wasn't 100% bullet-proof with exception stacks.

> > Also frankly often your analysis about what went wrong was just
> > incorrect.
> Still in denial, I see.
> Do you still claim that "the fallback position always did the right 
> thing"

No initially it was buggy and that caused several of the crashes.

> Despite the fact that the unwinder had sometimes *corrupted* the 
> incoming information so much that the fallback position was the one that 
> oopsed? And no, you didn't fix that.

No, it oopsed because it was broken by itself.
Anyways that got fixed quickly.

> And no, IT DID NOT use probe_kernel_address like you still claim.

There definitely was a patch that made it use it. You might have
not merged it though.
> Anyway, you work for Suse, I don't care what you do to the Suse kernel. 
> Maybe it will get stable some day. Somehow, I doubt it. 

So what is your proposed alternative to handle long backtraces? 
You didn't answer that question. Please do, I'm curious about your thoughts
in this area.


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