Re: [PATCH tip/core/rcu 41/55] rcu: Permit rt_mutex_unlock() with irqs disabled

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

On Sun, Sep 18, 2011 at 12:09:23PM +0800, Yong Zhang wrote:
> On Tue, Sep 06, 2011 at 11:00:35AM -0700, Paul E. McKenney wrote:
> > From: Paul E. McKenney <paul.mckenney@linaro.org>
> > 
> > Create a separate lockdep class for the rt_mutex used for RCU priority
> > boosting and enable use of rt_mutex_lock() with irqs disabled.  This
> > prevents RCU priority boosting from falling prey to deadlocks when
> > someone begins an RCU read-side critical section in preemptible state,
> > but releases it with an irq-disabled lock held.
> > 
> > Unfortunately, the scheduler's runqueue and priority-inheritance locks
> > still must either completely enclose or be completely enclosed by any
> > overlapping RCU read-side critical section.
> > 
> > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  kernel/rcutree_plugin.h |    6 ++++++
> >  kernel/rtmutex.c        |    8 ++++++++
> >  2 files changed, 14 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index d3127e8..f6c63ea 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -1149,6 +1149,8 @@ static void rcu_initiate_boost_trace(struct rcu_node *rnp)
> >  
> >  #endif /* #else #ifdef CONFIG_RCU_TRACE */
> >  
> > +static struct lock_class_key rcu_boost_class;
> > +
> >  /*
> >   * Carry out RCU priority boosting on the task indicated by ->exp_tasks
> >   * or ->boost_tasks, advancing the pointer to the next task in the
> > @@ -1211,10 +1213,14 @@ static int rcu_boost(struct rcu_node *rnp)
> >  	 */
> >  	t = container_of(tb, struct task_struct, rcu_node_entry);
> >  	rt_mutex_init_proxy_locked(&mtx, t);
> > +	/* Avoid lockdep false positives.  This rt_mutex is its own thing. */
> > +	lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
> > +				   "rcu_boost_mutex");
> >  	t->rcu_boost_mutex = &mtx;
> 
>   	raw_spin_unlock_irqrestore(&rnp->lock, flags);  <====A
> 
> >  	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
> >  	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
> > +	local_irq_restore(flags);
> 
> Does it help here?
> irq is enabled at A. So we still call rt_mutex_lock() with irq enabled.
> 
> Seems should s/raw_spin_unlock_irqrestore/raw_spin_unlock ?

Hmmm...  The above works at least by accident, but I am clearly not
testing calling rt_mutex_lock(&mtx) and rt_mutex_unlock(&mtx) with
interrupts disabled anywhere near as heavily as I thought I was.

I will fix this one way or the other.

> BTW, since we are in process context, 'flags' is not needed to save,
> no?

Only until the code gets moved/reused...

							Thanx, Paul

> Thanks,
> Yong
> 
> 
> >  
> >  	return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
> >  }
> > diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
> > index ab44911..2548f44 100644
> > --- a/kernel/rtmutex.c
> > +++ b/kernel/rtmutex.c
> > @@ -579,6 +579,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
> >  		    struct rt_mutex_waiter *waiter)
> >  {
> >  	int ret = 0;
> > +	int was_disabled;
> >  
> >  	for (;;) {
> >  		/* Try to acquire the lock: */
> > @@ -601,10 +602,17 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
> >  
> >  		raw_spin_unlock(&lock->wait_lock);
> >  
> > +		was_disabled = irqs_disabled();
> > +		if (was_disabled)
> > +			local_irq_enable();
> > +
> >  		debug_rt_mutex_print_deadlock(waiter);
> >  
> >  		schedule_rt_mutex(lock);
> >  
> > +		if (was_disabled)
> > +			local_irq_disable();
> > +
> >  		raw_spin_lock(&lock->wait_lock);
> >  		set_current_state(state);
> >  	}
> > -- 
> > 1.7.3.2
> > 
> > --
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