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

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Date:	Mon, 6 Jun 2011 15:10:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arne Jansen <lists@...-jansens.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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


* Ingo Molnar <mingo@...e.hu> wrote:

> So some essential piece of the puzzle is still missing.

Oh. I think the essential piece of the puzzle might be this code in 
printk():

asmlinkage int vprintk(const char *fmt, va_list args)
{
...
        lockdep_off();
        if (console_trylock_for_printk(this_cpu))
                console_unlock();

        lockdep_on();
...

So while i right now do not see how this (ancient) piece of logic 
causes trouble, could you try the patch below, does it make the 
WARN()+lockup go away?

Thanks,

	Ingo

diff --git a/kernel/printk.c b/kernel/printk.c
index 3518539..1b9d2be 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -859,7 +859,6 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 		zap_locks();
 	}
 
-	lockdep_off();
 	spin_lock(&logbuf_lock);
 	printk_cpu = this_cpu;
 
@@ -947,7 +946,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	 * Try to acquire and then immediately release the
 	 * console semaphore. The release will do all the
 	 * actual magic (print out buffers, wake up klogd,
-	 * etc). 
+	 * etc).
 	 *
 	 * The console_trylock_for_printk() function
 	 * will release 'logbuf_lock' regardless of whether it
@@ -956,7 +955,6 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	if (console_trylock_for_printk(this_cpu))
 		console_unlock();
 
-	lockdep_on();
 out_restore_irqs:
 	raw_local_irq_restore(flags);
 
--
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