linux-kernel - Re: [PATCH] module: Prevent recursion bug caused by module RCU check

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Date:	Tue, 20 Oct 2015 12:46:28 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] module: Prevent recursion bug caused by module RCU
 check

On Tue, 20 Oct 2015 18:39:52 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Oct 20, 2015 at 12:21:03PM -0400, Steven Rostedt wrote:
> > +++ b/kernel/module.c
> > @@ -284,11 +284,25 @@ static void module_assert_mutex(void)
> >  static void module_assert_mutex_or_preempt(void)
> >  {
> >  #ifdef CONFIG_LOCKDEP
> > +	static int once;
> > +
> >  	if (unlikely(!debug_locks))
> >  		return;
> >  
> > -	WARN_ON(!rcu_read_lock_sched_held() &&
> > -		!lockdep_is_held(&module_mutex));
> > +	/*
> > +	 * Would be nice to use WARN_ON_ONCE(), but the warning
> > +	 * that causes a stack trace may call __module_address()
> > +	 * which may call here, and we trigger the warning again,
> > +	 * before the WARN_ON_ONCE() updates its flag.
> > +	 * To prevent the recursion, we need to open code the
> > +	 * once logic.
> > +	 */
> > +	if (!once &&
> > +	    unlikely(!rcu_read_lock_sched_held() &&
> > +		     !lockdep_is_held(&module_mutex))) {
> > +		once++;
> 
> 		once = 1;
> 
> is more 'once' :-) Otherwise its once every 4-odd billion.

Once it's not 0, it never gets incremented again.

-- Steve

> 
> > +		WARN_ON(1);
> > +	}
> >  #endif
> >  }
> >  

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