linux-kernel - Re: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages

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Date:	Mon, 06 Jun 2011 15:25:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arne Jansen <lists@...-jansens.de>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	efault@....de, npiggin@...nel.dk, akpm@...ux-foundation.org,
	frank.rowand@...sony.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages

On Mon, 2011-06-06 at 12:26 +0200, Peter Zijlstra wrote:
> On Mon, 2011-06-06 at 12:00 +0200, Arne Jansen wrote:
> > As expected this apparently fixes the problem. But are we confident
> > enough this is the true source? If it's really that simple, printk
> > calling into the scheduler, why am I the only one seeing this?
> 
> Right, so apparently you have contention on console_sem and the up()
> actually does a wakeup. I'm still trying to figure out how to do that.

On a related note, I'm not quite sure what we need that
lockdep_{off,on}() for, I just build and booted a kernel without them
and so far life is good.

I tried lockdep splats and in-scheduler printk()s (although the latter
will still mess up the box if printk()'s up(console_sem) triggers a
wakeup for obvious reasons).

---
Subject: lockdep, printk: Remove lockdep_off from printk()
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date: Mon Jun 06 13:37:22 CEST 2011

Remove the lockdep_{off,on}() usage from printk() as it appears
superfluous, a kernel with this patch on can printk lockdep output just
fine.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
Index: linux-2.6/kernel/printk.c
===================================================================
--- linux-2.6.orig/kernel/printk.c
+++ linux-2.6/kernel/printk.c
@@ -838,7 +838,7 @@ asmlinkage int vprintk(const char *fmt,
 
 	preempt_disable();
 	/* This stops the holder of console_sem just where we want him */
-	raw_local_irq_save(flags);
+	local_irq_save(flags);
 	this_cpu = smp_processor_id();
 
 	/*
@@ -859,7 +859,6 @@ asmlinkage int vprintk(const char *fmt,
 		zap_locks();
 	}
 
-	lockdep_off();
 	spin_lock(&logbuf_lock);
 	printk_cpu = this_cpu;
 
@@ -956,9 +955,8 @@ asmlinkage int vprintk(const char *fmt,
 	if (console_trylock_for_printk(this_cpu))
 		console_unlock();
 
-	lockdep_on();
 out_restore_irqs:
-	raw_local_irq_restore(flags);
+	local_irq_restore(flags);
 
 	preempt_enable();
 	return printed_len;

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