linux-kernel - Re: [PATCH 3/3] printk: Avoid all wakeups from printk

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Message-ID: <20110609141959.GF21100@elte.hu>
Date:	Thu, 9 Jun 2011 16:19:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	efault@....de, Arne Jansen <lists@...-jansens.de>
Subject: Re: [PATCH 3/3] printk: Avoid all wakeups from printk


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Thu, 2011-06-09 at 15:55 +0200, Ingo Molnar wrote:
> 
> > > @@ -943,19 +939,14 @@ asmlinkage int vprintk(const char *fmt, 
> > >  		if (*p == '\n')
> > >  			new_text_line = 1;
> > >  	}
> > > +	printk_cpu = UINT_MAX;
> > > +	spin_unlock(&logbuf_lock);
> > 
> > Hm, is that printk_cpu setting inside the critical section safe? What 
> > happens if we get an NMI on this CPU in that window?
> 
> That's what the old code did, also NMI really shouldn't printk(), but
> yeah, can flip those two lines, but in that same vein I should also flip
> the setting of that variable with acquiring the lock.

Flipping it won't be enough - we could override another CPU's 
protection ...

Thanks,

	Ingo
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