Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings involving interrupt disabling

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Dec 06, 2011 at 11:27:26AM +0100, Peter Zijlstra wrote:
> On Tue, 2011-12-06 at 10:52 +0100, Peter Zijlstra wrote:
> > On Tue, 2011-12-06 at 09:26 +0800, Yong Zhang wrote:
> > 
> > > Yeah, because we call might_sleep() in rt_mutex_lock() unconditionally.
> > > But in this case the 'BUG: sleeping function called from invalid context
> > > at *' is obviously false positive.
> > 
> > Why can't this mutex acquisition not block?
> 
> Gaah!! I see, this 5342e269 patch is revolting.. guys that's really vile
> don't do that!
> 
> I tried reading the RCU code but I gave up.. rcu_boost() does:
> 
>   rt_mutex_init_proxy_locked();
>   raw_spin_unlock_irqrestore();
>   rt_mutex_lock();
>   rt_mutex_unlock();
> 
> vs rcu_read_unlock_special()'s RCU_READ_UNLOCK_BLOCKED branch:
> 
>   rt_mutex_unlock();
> 
> 
> The latter looks to be unbalanced because I can't actually find a
> matching lock. Also, all of that is ran with IRQs enabled. So what's the
> problem?

The rt_mutex_init_proxy_locked() creates the lock in held state,
held by the RCU reader who is holding up the grace period.
So rcu_read_unlock_special()'s rt_mutex_unlock() is balanced by the
rt_mutex_init_proxy_locked().

The problem with the IRQs enabled is the following sequence:

	rcu_read_lock();
	/* do stuff */
	local_irq_save(flags);
	/* do more stuff */
	rcu_read_unlock();
	/* do even more stuff */
	local_irq_restore(flags);

This has been legal in the past, and might well be used in places that
-rt does not exercise, hence the desire to explicitly legalize it.

							Thanx, Paul