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

Mathieu Desnoyers
2007-01-24T17:24:50Z

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@polymtl.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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