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SubjectRe: [PATCH] module: Prevent recursion bug caused by module RCU check
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 :-)


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