linux-kernel - Re: [PATCH] [2/2] Don't complain about disabled irqs when the system has paniced

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Date:	Tue, 02 Sep 2008 16:45:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	torvalds@...l.org, linux-kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH] [2/2] Don't complain about disabled irqs when the
	system has paniced

On Tue, 2008-09-02 at 16:40 +0200, Andi Kleen wrote:
> On Tue, Sep 02, 2008 at 04:28:03PM +0200, Peter Zijlstra wrote:
> > On Tue, 2008-09-02 at 15:49 +0200, Andi Kleen wrote:
> > > panic calls smp_send_stop which eventually calls smp_call_function_*.
> > > smp_call_function warns about disabled interrupts. But it's legal
> > > to call panic in this case. When this happens panic() prints
> > > several ugly backtraces. So don't check for disabled interrupts
> > > in panic state.
> > 
> > While it might be legal for panic to be called from such contexts, I
> > understand those warnings are there to warn of deadlocks.
> > 
> > So with the below patch you allow panic to deadlock if I understand
> > things correctly.
> 
> Please describe the deadlock exactly.  I don't think it can deadlock 
> in this case. 

Then why are those warnings there? The deadlock is for the CSD_FLAG_WAIT
case, which can always happen due to the static csd data fallback.

The deadlock scenario is long the lines of two such smp_call_function*()
both under irq disabled calling each other with CSD_FLAG_WAIT set.
Neither remote cpu will handle the IPI due to irqs being disabled, so
we'll wait at-infinitum for completion.

> Besides do you prefer to not allow panic from interrupts/machine
> checks etc. anymore? 

However did I imply that, I just said your fix looked iffy.

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