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

[Yong Zhang]

On Tue, Sep 06, 2011 at 11:00:35AM -0700, Paul E. McKenney wrote:
> From: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  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 ?

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

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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