Re: [PATCH,RFC] Add call_rcu_sched()

From: Andrew Morton
Date: Tue Apr 08 2008 - 05:15:14 EST


On Tue, 8 Apr 2008 14:26:37 +0530 Gautham R Shenoy <ego@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

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