linux-kernel - Re: mm,console: circular dependency between console

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Date:	Mon, 2 Jun 2014 21:55:29 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Jan Kara <jack@...e.cz>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: mm,console: circular dependency between console_sem and zone
 lock

On Sun, 01 Jun 2014 10:08:21 -0400
Sasha Levin <sasha.levin@...cle.com> wrote:

> On 05/12/2014 12:28 PM, Jan Kara wrote:
> > On Wed 07-05-14 22:03:08, Sasha Levin wrote:
> >> > Hi all,
> >> > 
> >> > While fuzzing with trinity inside a KVM tools guest running the latest -next
> >> > kernel I've stumbled on the following spew:
> >   Thanks for report. So the problem seems to be maginally valid but I'm not
> > 100% sure whom to blame :). So printk() code calls up() which calls
> > try_to_wake_up() under console_sem.lock spinlock. That function can take
> > rq->lock which is all expected.
> > 
> > The next part of the chain is that during CPU initialization we call
> > __sched_fork() with rq->lock which calls into hrtimer_init() which can
> > allocate memory which creates a dependency rq->lock => zone.lock.rlock.
> > 
> > And memory management code calls printk() which zone.lock.rlock held which
> > closes the loop. Now I suspect the second link in the chain can happen only
> > while CPU is booting and might even happen only if some debug options are
> > enabled. But I don't really know scheduler code well enough. Steven?
> 
> I've cc'ed Peter and Ingo who may be able to answer that, as it still happens
> on -next.
> 

Hmm, it failed on a try lock, but on the spinlock within the trylock. I
wonder if we should add this.

Peter?

-- Steve

diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 6815171..6579f84 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -132,7 +132,9 @@ int down_trylock(struct semaphore *sem)
 	unsigned long flags;
 	int count;
 
-	raw_spin_lock_irqsave(&sem->lock, flags);
+	if (!raw_spin_trylock_irqsave(&sem->lock, flags))
+		return 1;
+
 	count = sem->count - 1;
 	if (likely(count >= 0))
 		sem->count = count;
--
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