linux-kernel - [PATCH] order of lockdep off/on in vprintk() should be changed

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Date:	Wed, 24 Jan 2007 12:24:45 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Andrew Morton <akpm@...l.org>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>,
	Richard J Moore <richardj_moore@...ibm.com>,
	linux-kernel@...r.kernel.org,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Christoph Hellwig <hch@...radead.org>,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	Ingo Molnar <mingo@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] order of lockdep off/on in vprintk() should be changed

The order of locking between lockdep_off/on() and local_irq_save/restore() in
vprintk() should be changed.
  
* In kernel/printk.c :

vprintk() does : 

preempt_disable()
local_irq_save()
lockdep_off()
spin_lock(&logbuf_lock)
spin_unlock(&logbuf_lock)
if(!down_trylock(&console_sem))
   up(&console_sem)
lockdep_on()
local_irq_restore() 
preempt_enable()
    
The goals here is to make sure we do not call printk() recursively from
kernel/lockdep.c:__lock_acquire() (called from spin_* and down/up) nor from
kernel/lockdep.c:trace_hardirqs_on/off() (called from local_irq_restore/save).
It can then potentially call printk() through mark_held_locks/mark_lock.
  
It correctly protects against the spin_lock call and the up/down call, but it
does not protect against local_irq_restore. It could cause infinite recursive
printk/trace_hardirqs_on() calls when printk() is called from the
mark_lock() error handing path.

We should change the locking so it becomes correct :

preempt_disable()
lockdep_off()
local_irq_save()
spin_lock(&logbuf_lock)
spin_unlock(&logbuf_lock)
if(!down_trylock(&console_sem))
   up(&console_sem)
local_irq_restore()
lockdep_on()
preempt_enable()


Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>

--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -530,8 +530,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 		zap_locks();
 
 	/* This stops the holder of console_sem just where we want him */
-	local_irq_save(flags);
 	lockdep_off();
+	local_irq_save(flags);
 	spin_lock(&logbuf_lock);
 	printk_cpu = smp_processor_id();
 
@@ -640,8 +640,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 			console_locked = 0;
 			up(&console_sem);
 		}
-		lockdep_on();
 		local_irq_restore(flags);
+		lockdep_on();
 	} else {
 		/*
 		 * Someone else owns the drivers.  We drop the spinlock, which
@@ -650,8 +650,8 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 		 */
 		printk_cpu = UINT_MAX;
 		spin_unlock(&logbuf_lock);
-		lockdep_on();
 		local_irq_restore(flags);
+		lockdep_on();
 	}
 
 	preempt_enable();
-- 
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