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

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Date:	Mon, 06 Jun 2011 17:47:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arne Jansen <lists@...-jansens.de>,
	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 17:09 +0200, Peter Zijlstra wrote:
> On Mon, 2011-06-06 at 16:58 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Sun, 2011-06-05 at 17:13 +0200, Ingo Molnar wrote:
> > > 
> > > > Now, this patch alone just removes a debugging check - but i'm not 
> > > > sure the debugging check is correct - we take the pi_lock in a raw 
> > > > way - which means it's not lockdep covered.
> > > 
> > > Ever since tglx did s/raw_/arch_/g raw_ is covered by lockdep.
> > 
> > It's not lockdep covered due to the lockdep_off(), or am i missing 
> > something?
> 
> Your initial stmt was about the raw_ part, raw_ locks are tracked by
> lockdep ever since tglx renamed them to arch_ and introduced new raw_
> primitives.
> 
> But yeah, the lockdep_off() stuff also disables all tracking, on top of
> that it also makes lock_is_held() return an unconditional false (even if
> the lock was acquired before lockdep_off and thus registered).
> 
> My patch that fixes lock_is_held() should avoid false
> lockdep_assert_held() explosions and this this printk() while rq->lock
> problem.
> 
> Removing lockdep_off() usage from printk() would also be nice, but Mike
> triggered logbuf_lock <-> rq->lock inversion with that due to the
> up(&console_sem) wakeup muck.
> 
> Ideally we'd pull the up() out from under logbuf_lock, am looking at
> that.

something like so,.. but then there's a comment about console_sem and
logbuf_lock interlocking in interating ways, but it fails to mention how
and why. But I think it should maybe work.. 

Needs more staring at, preferably by someone who actually understands
that horrid mess :/ Also, this all still doesn't make printk() work
reliably while holding rq->lock.

---
Index: linux-2.6/kernel/printk.c
===================================================================
--- linux-2.6.orig/kernel/printk.c
+++ linux-2.6/kernel/printk.c
@@ -686,6 +686,7 @@ static void zap_locks(void)
 
 	oops_timestamp = jiffies;
 
+	debug_locks_off();
 	/* If a crash is occurring, make sure we can't deadlock */
 	spin_lock_init(&logbuf_lock);
 	/* And make sure that we print immediately */
@@ -782,7 +783,7 @@ static inline int can_use_console(unsign
 static int console_trylock_for_printk(unsigned int cpu)
 	__releases(&logbuf_lock)
 {
-	int retval = 0;
+	int retval = 0, wake = 0;
 
 	if (console_trylock()) {
 		retval = 1;
@@ -795,12 +796,14 @@ static int console_trylock_for_printk(un
 		 */
 		if (!can_use_console(cpu)) {
 			console_locked = 0;
-			up(&console_sem);
+			wake = 1;
 			retval = 0;
 		}
 	}
 	printk_cpu = UINT_MAX;
 	spin_unlock(&logbuf_lock);
+	if (wake)
+		up(&console_sem);
 	return retval;
 }
 static const char recursion_bug_msg [] =
@@ -836,9 +839,8 @@ asmlinkage int vprintk(const char *fmt,
 	boot_delay_msec();
 	printk_delay();
 
-	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 +861,6 @@ asmlinkage int vprintk(const char *fmt,
 		zap_locks();
 	}
 
-	lockdep_off();
 	spin_lock(&logbuf_lock);
 	printk_cpu = this_cpu;
 
@@ -956,11 +957,9 @@ 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;
 }
 EXPORT_SYMBOL(printk);
@@ -1271,8 +1270,8 @@ void console_unlock(void)
 	if (unlikely(exclusive_console))
 		exclusive_console = NULL;
 
-	up(&console_sem);
 	spin_unlock_irqrestore(&logbuf_lock, flags);
+	up(&console_sem);
 	if (wake_klogd)
 		wake_up_klogd();
 }

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