Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 23 Feb 2009 08:17:26 -0800 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Tue, Feb 24, 2009 at 12:29:36AM +1100, Nick Piggin wrote:
> > On Monday 23 February 2009 16:17:09 Paul E. McKenney wrote:
> > 
> > > The boot CPU runs in the context of its idle thread during boot-up.
> > > During this time, idle_cpu(0) will always return nonzero, which will
> > > fool Classic and Hierarchical RCU into deciding that a large chunk of
> > > the boot-up sequence is a big long quiescent state.  This in turn causes
> > > RCU to prematurely end grace periods during this time.
> > >
> > > This patch creates a new global variable that is set to 1 just before
> > > the boot CPU first enters the scheduler, after which the idle task
> > > really is idle.
> > 
> > Nice work all (btw. if this patch goes in rather than using system_state,
> > then please make the variable __read_mostly).
> 
> Hmmm...  I misread this and made system_state be __read_mostly.  Let
> me know if this is bad, easy to fix if needed.

Please don't use system_state.  The whole thing is just bad design. 
It's a global variable, breaks encapsulation, creates interactions etc.
CS-101 stuff.