linux-kernel - Re: [tip:core/debug] debug lockups: Improve lockup detection

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Date:	Sun, 2 Aug 2009 22:41:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	paulmck@...ux.vnet.ibm.com, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/debug] debug lockups: Improve lockup detection

* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Sun, 2 Aug 2009 21:26:57 +0200 Ingo Molnar <mingo@...e.hu> wrote:
> 
> > > I think this just broke all non-x86 non-sparc SMP architectures.
> > 
> > Yeah - it 'broke' them in the sense of them not having a working 
> > trigger_all_cpu_backtrace() implementation to begin with.
> 
> c'mon.  It broke them in the sense that sysrq-l went from "works" 
> to "doesn't work".

You are right (i broke it with my patch) but the thing is, sysrq-l 
almost useless currently: it uses schedule_work() which assumes a 
mostly working system with full irqs and scheduling working fine. 
Now, i dont need sysrq-l on mostly working systems.

So the 'breakage' is of something that was largely useless: and now 
you put the onus of implementing it for _all_ architectures (which i 
dont use) on me?

If that's the requirement then i'll have to keep this as a local 
debug hack and not do an upstream solution - i dont have the 
resources to do it for all ~10 SMP architectures.

sysrq-l has been messed up really and now that messup limits the 
adoption of the much more useful solution? I didnt make this thing 
up, i tried to use it on a locked up system and wondered why it 
emits nothing and why it uses a separate facility instead of an 
existing trigger-backtraces facility (which the spinlock-debug code 
uses).

> It would take months for the relevant arch maintainers to even 
> find out about this, after which they're left with dud kernels out 
> in the field.
> 
> It's better to break the build or to emit warnings than to 
> silently and secretly break their stuff.

But that warning will bounce the ball back to me, wont it? My patch 
will be blamed for 'breaking' those architectures, right?

	Ingo
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