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

[Thomas Gleixner <tglx@linutronix.de>]

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?

Thanks,

	tglx