Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

From: Paul E. McKenney
Date: Wed Jan 13 2016 - 11:23:20 EST


On Wed, Jan 13, 2016 at 10:05:49AM +0100, Thomas Gleixner wrote:
> Sasha,
>
> On Tue, 12 Jan 2016, Sasha Levin wrote:
>
> Cc'ing Paul, Peter
>
> > While fuzzing with trinity inside a KVM tools guest, running the latest -next
> > kernel, I've hit the following lockdep warning:
>
> > [ 3408.474461] Possible interrupt unsafe locking scenario:
> >
> > [ 3408.474461]
> >
> > [ 3408.475239] CPU0 CPU1
> >
> > [ 3408.475809] ---- ----
> >
> > [ 3408.476380] lock(&lock->wait_lock);
> >
> > [ 3408.476925] local_irq_disable();
> >
> > [ 3408.477640] lock(&(&new_timer->it_lock)->rlock);
> >
> > [ 3408.478607] lock(&lock->wait_lock);
>
> That comes from rcu_read_unlock:
>
> rcu_read_unlock()
> rcu_read_unlock_special()
> ...
> rt_mutex_unlock(&rnp->boost_mtx);
> raw_spin_lock(&boost_mtx->wait_lock);
>
> > [ 3408.479445] <Interrupt>
> >
> > [ 3408.479796] lock(&(&new_timer->it_lock)->rlock);
>
> So the task on CPU0 holds rnp->boost_mtx.wait_lock and then the interrupt
> deadlocks on the timer->it_lock.
>
> We can fix that particular issue in the posix-timer code by making the
> locking symetric:
>
> rcu_read_lock();
> spin_lock_irq(timer->lock);
>
> ...
>
> spin_unlock_irq(timer->lock);
> rcu_read_unlock();
>
> instead of:
>
> rcu_read_lock();
> spin_lock_irq(timer->lock);
> rcu_read_unlock();
>
> ...
>
> spin_unlock_irq(timer->lock);
>
> But the question is, whether this is the only offending code path in tree. We
> can avoid the hassle by making rtmutex->wait_lock irq safe.
>
> Thoughts?

Given that the lock is disabling irq, I don't see a problem with
extending the RCU read-side critical section to cover the entire
irq-disabled region. Your point about the hassle of finding and fixing
all the other instances of this sort is well taken, however.

Thanx, Paul