linux-kernel - Re: [patch 1/3] printk: fix wake_up

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Date:	Fri, 26 Nov 2010 13:13:25 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Christof Schmitt <christof.schmitt@...ibm.com>,
	Frank Blaschka <frank.blaschka@...ibm.com>,
	Horst Hartmann <horsth@...ux.vnet.ibm.com>
Subject: Re: [patch 1/3] printk: fix wake_up_klogd() vs cpu hotplug

On Fri, Nov 26, 2010 at 01:10:08PM +0100, Peter Zijlstra wrote:
> On Fri, 2010-11-26 at 13:00 +0100, Heiko Carstens wrote:
> > plain text document attachment (001_printk_preempt.diff)
> > From: Heiko Carstens <heiko.carstens@...ibm.com>
> > 
> > wake_up_klogd() may get called from preemtible context but uses
> > __raw_get_cpu_var() to write to a per cpu variable. If it gets preempted between
> > getting the address and writing to it, the cpu in question could be offline if
> > the process gets scheduled back and hence writes to the per cpu data of an offline
> > cpu.
> > 
> > No idea why that behaviour was introduced with fa33507a "printk: robustify
> > printk, fix #2" which was supposed to fix a "using smp_processor_id() in
> > preemptible" warning.
> > 
> > Let's use get_cpu_var() instead which disables preemption and makes sure that
> > the outlined scenario cannot happen.
> > 
> > Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
> > ---
> >  kernel/printk.c |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > --- a/kernel/printk.c
> > +++ b/kernel/printk.c
> > @@ -1087,8 +1087,10 @@ int printk_needs_cpu(int cpu)
> >  
> >  void wake_up_klogd(void)
> >  {
> > -	if (waitqueue_active(&log_wait))
> > -		__raw_get_cpu_var(printk_pending) = 1;
> > +	if (waitqueue_active(&log_wait)) {
> > +		get_cpu_var(printk_pending) = 1;
> > +		put_cpu_var(printk_pending);
> > +	}
> >  }
> >  
> >  /**
> > 
> 
> But but but, the cpu can still be offlined between writing this state
> and the next tick happening, right?

Yes, that's what the second patch would fix as a side effect.
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