linux-kernel - Re: [PATCH,RFC] Add call_rcu

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Date:	Tue, 8 Apr 2008 10:17:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	ego@...ibm.com, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...ymtl.ca, mingo@...e.hu, hch@...radead.org,
	mmlnx@...ibm.com, dipankar@...ibm.com, dsmith@...hat.com,
	rostedt@...dmis.org, adrian.bunk@...ial.fi, a.p.zijlstra@...llo.nl,
	niv@...ibm.com, dvhltc@...ibm.com, rusty@....ibm.com,
	jkenisto@...ux.vnet.ibm.com, oleg@...sign.ru
Subject: Re: [PATCH,RFC] Add call_rcu_sched()

On Tue, Apr 08, 2008 at 02:07:58AM -0700, Andrew Morton wrote:
> On Tue, 8 Apr 2008 14:26:37 +0530 Gautham R Shenoy <ego@...ibm.com> wrote:
> 
> > On Tue, Apr 08, 2008 at 01:39:36AM -0700, Andrew Morton wrote:
> > > On Tue, 8 Apr 2008 13:40:48 +0530 Gautham R Shenoy <ego@...ibm.com> wrote:
> > > 
> > > > > > +			rdp = RCU_DATA_CPU(cpu);
> > > >                                                                <-- here ------\
> > > > > > +			spin_lock_irqsave(&rdp->lock, flags);                   |
> > > > > > +                                                                         |
> > > > > > +			/*                                                      |
> > > > > > +			 * We are running on this CPU irq-disabled, so no       |
> > > > > > +			 * CPU can go offline until we re-enable irqs.          |
> > > > >                                                                             |
> > > > > but, but, but.  The cpu at `cpu' could have gone offline just before we     |
> > > > > disabled local interrupts.                                                  |
> > > >                                                                               |
> > > > In that case the CPU_DEAD callback should have migrated the rcu-lists to      |
> > > > a cpu which is online.                                                        |
> > >                                                                                 |
> > > But local variable rdp might be pointing at the now-offlined CPU's data? -------/
> > 
> > Right. But then rdp wouldn't contain anything useful at this point.
> > So, we may only end up taking the rdp->lock, observe that there's nothing to do,
> > and move on.
> > 
> > Is there something else that I am missing?
> 
> erm, I guess that'll work OK.
> 
> There were intentions to release the per-cpu memory during unplug, but
> nobody has threatened to do that for a while.

That would be a bit painful in a number of situations...  :-/

							Thanx, Paul
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