Subject: Re: Fw: 2.6.16 crashes when running numastat on p575

From: Andrew Morton <[email-masked]>
Date: Mon, 3 Apr 2006 14:18:34 -0700

Christoph Lameter <[email-masked]> wrote:
>
> On Mon, 3 Apr 2006, Nathan Lynch wrote:
> 
> > > There are many other for_each_*_cpu loops in the kernel that do not have 
> > > any of the instrumentation you suggest. I suggest you come up with a 
> > > general solution and then go through all of them and fix this. Please be 
> > > aware that many of these loops are performance critical.
> > 
> > But this one isn't, right?
> 
> Right. One could use more expensive processing here.

Hopefully none of the for_each_foo() loops are performance-critical - those
things are expensive.

> > And I'm afraid there's a misunderstanding here -- only
> > for_each_online_cpu (or accessing the cpu online map in general) has
> > such restrictions -- for_each_possible_cpu doesn't require any locking
> > or preempt tricks since cpu_possible_map must not change after boot.

for_each_present_cpu() presumably has the same problems.

> Correct. We may want to audit the kernel and check that each 
> for_each_possible_cpu or for_each_cpu is really correct.

A fair bit of that has been happening in recent weeks.

But yes, we should be protecting these things with rcu_read_lock() if
possible, lock_cpu_hotplug() otherwise.

(rcu_read_lock() might not be the appropriate name for this operation -
maybe it should be an open-coded preempt_disable().  Or some other suitably
named alias; dunno).