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SubjectRe: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages
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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@infradead.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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