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

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Date:	Mon, 06 Jun 2011 11:01:49 +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 Sun, 2011-06-05 at 22:15 +0200, Arne Jansen wrote:
> 
> Can lockdep just get confused by the lockdep_off/on calls in printk
> while scheduling is allowed? There aren't many users of lockdep_off().

Yes!, in that case lock_is_held() returns false, triggering the warning.
I guess there's an argument to be made in favour of the below..

---
 kernel/lockdep.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 53a6895..e4129cf 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -3242,7 +3242,7 @@ int lock_is_held(struct lockdep_map *lock)
 	int ret = 0;
 
 	if (unlikely(current->lockdep_recursion))
-		return ret;
+		return 1; /* avoid false negative lockdep_assert_held */
 
 	raw_local_irq_save(flags);
 	check_flags(flags);

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