linux-kernel - Re: [PATCH] powerpc - Initialize the irq radix tree earlier

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Message-Id: <1217509104.19050.11.camel@localhost>
Date:	Thu, 31 Jul 2008 22:58:24 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	Sebastien Dugue <sebastien.dugue@...l.net>
Cc:	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org, benh@...nel.crashing.org,
	paulus@...ba.org, jean-pierre.dion@...l.net,
	gilles.carry@....bull.net, tinytim@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org
Subject: Re: [PATCH] powerpc - Initialize the irq radix tree earlier

On Thu, 2008-07-31 at 14:00 +0200, Sebastien Dugue wrote:
> On Thu, 31 Jul 2008 21:40:56 +1000 Michael Ellerman <michael@...erman.id.au> wrote:
> > 
> > This boot ordering stuff is pretty hairy, so I might have missed
> > something, but this is how the code is ordered AFAICT:
> > 
> > start_kernel()
> > 	init_IRQ()
> > 	...
> > 	local_irq_enable()
> > 	...
> > 	rest_init()
> > 		kernel_thread()
> > 			kernel_init()
> > 				smp_prepare_cpus()
> > 					smp_xics_probe()	(via smp_ops->probe())
> > 
> > 
> > What's stopping us from taking an irq between local_irq_enable() and
> > smp_xics_probe() ?  Is it just that no one's request_irq()'ed them yet?
> 
>   It's hairy, I agree, but as you've mentioned no one has done a request_irq()
> at that point. The first one to do it is smp_xics_probe() for the IPI.

Hmm, I don't think that's strong enough. I can trivially cause irqs to
fire during a kexec reboot just by mashing the keyboard.

And during a kdump boot all sorts of stuff could be firing. Even during
a clean boot, from firmware, I don't think we can guarantee that
nothing's going to fire.

.. after a bit of testing ..

It seems it actually works (sort of). 

xics_remap_irq() calls irq_radix_revmap_lookup(), which calls:

ptr = radix_tree_lookup(&host->revmap_data.tree, hwirq);

And because host->revmap_data.tree was zalloc'ed we trip on the first
check here:



cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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