linux-kernel - Re: [PATCH] ia64: prevent irq migration race in __cpu

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Message-ID: <20090206231709.GP10918@linux.vnet.ibm.com>
Date:	Fri, 6 Feb 2009 15:17:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alex Chiang <achiang@...com>, "Luck, Tony" <tony.luck@...el.com>,
	"stable@...nel.org" <stable@...nel.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ia64: prevent irq migration race in __cpu_disable path

On Fri, Feb 06, 2009 at 02:05:50PM -0700, Alex Chiang wrote:
> * Luck, Tony <tony.luck@...el.com>:
> > > This is wrong because fixup_irqs calls migrate_irqs, and in
> > > migrate_irqs, we use the cpu_online_map to:
> > >
> > >       1. look for interrupts on current CPU
> > >       2. if we find one, move it to the first available CPU in
> > >       the cpu_online_map
> > >
> > > This means we can potentially migrate an interrupt off ourself
> > > back to... ourself. Uh oh.
> > 
> > Should we make migrate_irqs smarter then ... does any caller really
> > expect that it would "migrate" the irq to the same cpu?
> 
> The only thing migrate_irqs does is locate irqs that have their
> CPU affinity set to the current CPU, and if so, changes the
> affinity.
> 
> We still have possible pending timer interrupts that we need to
> handle, so I'm not seeing how changing the migrate_irqs
> implementation (to avoid migrating to ourself) will handle that.
> 
> On one hand, I think the only irq handler that can be called at
> this point is our timer_interrupt, which doesn't seem to be using
> any RCU APIs.
> 
> On the other hand, if we really want to make sure that we're not
> calling interrupt handlers with our CPU marked as 'offline', then
> we need to fix ia64_process_pending_intr() so that we're not
> firing our timer_interrupt with a NULL pt_regs.
> 
> Add in to the mix that x86 seems to have the same assumption that
> we do (that it's ok to call our timer interrupt handler even if
> we're already marked as 'offline').
> 
> I'm leaning towards reverting Paul's previous commit entirely
> because
> 
> 	- migrate_platform_irqs() doesn't cause any interrupt
> 	  handlers to be fired by itself
> 
> 	- it also uses cpu_online)map to find a new CPU to assign
> 	  CPEI to
> 
> Thoughts?

I would suggest at least a comment stating why it is safe to take the
interrupts on a CPU marked offline.  As to the eventual solution, you
guys are the experts on your architecture.

							Thanx, Paul
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