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

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Message-Id: <20090223011712.853aa94d.akpm@linux-foundation.org>
Date:	Mon, 23 Feb 2009 01:17:12 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Vegard Nossum <vegard.nossum@...il.com>, stable@...nel.org,
	Nick Piggin <npiggin@...e.de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

On Mon, 23 Feb 2009 10:07:35 +0100 Ingo Molnar <mingo@...e.hu> wrote:

> > ---
> > 
> >  init/main.c         |    3 +++
> >  kernel/rcuclassic.c |    4 +++-
> >  kernel/rcutree.c    |    4 +++-
> >  3 files changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/init/main.c b/init/main.c
> > index 8442094..51f4b71 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -121,6 +121,8 @@ static char *static_command_line;
> >  static char *execute_command;
> >  static char *ramdisk_execute_command;
> >  
> > +int idle_task_is_really_idle;	/* set to 1 late in boot. */
> > +
> >  #ifdef CONFIG_SMP
> >  /* Setup configured maximum number of CPUs to activate */
> >  unsigned int __initdata setup_max_cpus = NR_CPUS;
> > @@ -463,6 +465,7 @@ static noinline void __init_refok rest_init(void)
> >  	 * at least once to get things moving:
> >  	 */
> >  	init_idle_bootup_task(current);
> > +	idle_task_is_really_idle = 1;
> >  	preempt_enable_no_resched();
> >  	schedule();
> >  	preempt_disable();
> 
> Could you please use system_state instead? We could insert a new 
> stage - or just use SYSTEM_RUNNING as the trigger.

I think the standalone flag is better (once those extern-decls-in-C get
fixed).

system_state's semantics have, err, evolved over time.  If this happens
again (and the patch sneaks past my attention) then there's a risk that
code which depends upon system_state will break - this has happened in
the past.  Plus piling more dependencies on system_state of course
makes any evolution of its semantics harder to do...

It seems safer/saner/more-robust to create the additional
application-specific global as Paul has done.  One more word of bss
won't kill us.  It adds a few more words of .text, but it's in __init.


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