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Date: Tue, 20 Oct 2015 18:56:49 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH] module: Prevent recursion bug caused by module RCU check
Message-ID: <20151020165649.GJ17308@twins.programming.kicks-ass.net>
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 <20151020163952.GI17308@twins.programming.kicks-ass.net>
 <20151020124628.726ef8bf@gandalf.local.home>
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On Tue, Oct 20, 2015 at 12:46:28PM -0400, Steven Rostedt wrote:
> On Tue, 20 Oct 2015 18:39:52 +0200
> Peter Zijlstra <peterz@infradead.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.

*sigh*, so much for pretending to be awake :-)