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

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Message-Id: <1217511566.19050.26.camel@localhost>
Date:	Thu, 31 Jul 2008 23:39:26 +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 15:26 +0200, Sebastien Dugue wrote:
> On Thu, 31 Jul 2008 23:01:39 +1000 Michael Ellerman <michael@...erman.id.au> wrote:
> 
> > On Thu, 2008-07-31 at 22:58 +1000, Michael Ellerman wrote:
> > > 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:
> > 
> > @#$% ctrl-enter == send!
> > 
> > Continuing ...
> > 
> > void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
> > {
> >         unsigned int height, shift;
> >         struct radix_tree_node *node, **slot;
> > 
> >         node = rcu_dereference(root->rnode);
> >         if (node == NULL)
> >                 return NULL;
> > 
> > Which means irq_radix_revmap_lookup() will return NO_IRQ, which is cool.
> 
>   Which is what I intended so that as long as no IRQ is registered we
> return NO_IRQ.
> 
> > 
> > 
> > So I think it can fly, as long as we're happy that we can't reverse map
> > anything until smp_xics_probe() - and I think that's true, as any irq we
> > take will be invalid.
> 
>   That's true as no IRQs are registered before smp_xics_probe() and for any
> interrupt we might get before that, irq_radix_revmap_lookup() will return
> NO_IRQ.

Cool, we agree :) 

My only worry is that we might be relying on on the particular radix
tree implementation a bit too much. Is it documented somewhere that
the /very/ first check is for root->rnode != NULL, and the rest of the
root may be unintialised?

And I think it needs a big fat comment in the irq code saying that it's
safe because revmap_data is zalloc'ed, and that means the radix lookup
will fail (safely).

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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