linux-kernel - Re: [PATCH] prevent powerpc from invoking irq handlers on offline CPUs

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Message-ID: <20080901050003.GN7015@linux.vnet.ibm.com>
Date:	Sun, 31 Aug 2008 22:00:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-kernel@...r.kernel.org, paulus@...ba.org,
	jwboyer@...ux.vnet.ibm.com, tnt@...tNt.com,
	grant.likely@...retlab.ca, linuxppc-dev@...abs.org,
	manfred@...orfullife.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] prevent powerpc from invoking irq handlers on offline
	CPUs

On Mon, Sep 01, 2008 at 01:14:40PM +1000, Benjamin Herrenschmidt wrote:
> On Sun, 2008-08-31 at 19:06 -0700, Paul E. McKenney wrote:
> > On Mon, Sep 01, 2008 at 10:34:44AM +1000, Benjamin Herrenschmidt wrote:
> > > On Sun, 2008-08-31 at 10:31 -0700, Paul E. McKenney wrote:
> > > > Make powerpc refrain from clearing a given to-be-offlined CPU's bit in the
> > > > cpu_online_mask until it has processed pending irqs.  This change
> > > > prevents other CPUs from being blindsided by an apparently offline CPU
> > > > nevertheless changing globally visible state.
> > > > 
> > > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > ---
> > > 
> > > Sounds reasonable... the only possible worry here is if somebody tries
> > > an IPI ... The IPI code will and the target CPU mask with the online
> > > map, so it may try to send to the to-be-offlined CPU and timeout, no ?
> > 
> > OK.  Do we need separate IPI and online masks?
> 
> Shouldn't we already have routed all interrupts to other CPUs anyway ?
> 
> IE. The affinity of all interrupts should have been updated. So the
> only thing we're going to get here are possibly IPIs and decrementer, 
> I don't see it being a big deal making sure we test we are online when
> receiving it.

It did look to me that the CPU removed itself from the interrupt queue
before re-enabling interrupts, so makes sense to me...

							Thanx, Paul
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