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

Michael Ellerman
2008-07-31T13:01:54Z

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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@ellerman.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.

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.

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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