Re: [debug patch] printk: Add a printk killswitch to robustify NMIwatchdog messages

From: Peter Zijlstra
Date: Mon Jun 06 2011 - 09:26:05 EST

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@xxxxxxxxx>
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

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
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,

/* 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,

- lockdep_off();
printk_cpu = this_cpu;

@@ -956,9 +955,8 @@ asmlinkage int vprintk(const char *fmt,
if (console_trylock_for_printk(this_cpu))

- lockdep_on();
- raw_local_irq_restore(flags);
+ local_irq_restore(flags);

return printed_len;

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