Date:	Thu, 5 Sep 2013 11:22:55 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Lai Jiangshan <eag0628@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 5/8] rcu: eliminate deadlock for rcu read site


Sorry for taking so long to review. So many other things to do :-/

On Fri, 23 Aug 2013 14:26:39 +0800
Lai Jiangshan <laijs@...fujitsu.com> wrote:

> [PATCH] rcu/rt_mutex: eliminate a kind of deadlock for rcu read site

"rcu read site"?

This is specific to boosting, thus boosting should be in the subject,
perhaps something like:

"Eliminate deadlock due to rcu boosting"

?

> 
> Current rtmutex's lock->wait_lock doesn't disables softirq nor irq, it will
> cause rcu read site deadlock when rcu overlaps with any softirq-context/irq-context lock.
> 
> @L is a spinlock of softirq or irq context.
> 
> CPU1					cpu2(rcu boost)
> rcu_read_lock()				rt_mutext_lock()
> <preemption and reschedule back>	  raw_spin_lock(lock->wait_lock)
> spin_lock_XX(L)				  <interrupt and doing softirq or irq>
> rcu_read_unlock()                         do_softirq()
>   rcu_read_unlock_special()
>     rt_mutext_unlock()
>       raw_spin_lock(lock->wait_lock)	    spin_lock_XX(L)  **DEADLOCK**
> 
> This patch fixes this kind of deadlock by removing rt_mutext_unlock() from
> rcu_read_unlock(), new rt_mutex_rcu_deboost_unlock() is called instead.
> Thus rtmutex's lock->wait_lock will not be called from rcu_read_unlock().
> 
> This patch does not eliminate all kinds of rcu-read-site deadlock,
> if @L is a scheduler lock, it will be deadlock, we should apply Paul's rule
> in this case.(avoid overlapping or preempt_disable()).
> 
> rt_mutex_rcu_deboost_unlock() requires the @waiter is queued, so we
> can't directly call rt_mutex_lock(&mtx) in the rcu_boost thread,
> we split rt_mutex_lock(&mtx) into two steps just like pi-futex.
> This result a internal state in rcu_boost thread and cause
> rcu_boost thread a bit more complicated.
> 
> Thanks
> Lai
> 
> diff --git a/include/linux/init_task.h b/include/linux/init_task.h
> index 5cd0f09..8830874 100644
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -102,7 +102,7 @@ extern struct group_info init_groups;
>  
>  #ifdef CONFIG_RCU_BOOST
>  #define INIT_TASK_RCU_BOOST()						\
> -	.rcu_boost_mutex = NULL,
> +	.rcu_boost_waiter = NULL,
>  #else
>  #define INIT_TASK_RCU_BOOST()
>  #endif
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index e9995eb..1eca99f 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1078,7 +1078,7 @@ struct task_struct {
>  	struct rcu_node *rcu_blocked_node;
>  #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  #ifdef CONFIG_RCU_BOOST
> -	struct rt_mutex *rcu_boost_mutex;
> +	struct rt_mutex_waiter *rcu_boost_waiter;
>  #endif /* #ifdef CONFIG_RCU_BOOST */
>  
>  #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> @@ -1723,7 +1723,7 @@ static inline void rcu_copy_process(struct task_struct *p)
>  	p->rcu_blocked_node = NULL;
>  #endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>  #ifdef CONFIG_RCU_BOOST
> -	p->rcu_boost_mutex = NULL;
> +	p->rcu_boost_waiter = NULL;
>  #endif /* #ifdef CONFIG_RCU_BOOST */
>  	INIT_LIST_HEAD(&p->rcu_node_entry);
>  }
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 769e12e..d207ddd 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -33,6 +33,7 @@
>  #define RCU_KTHREAD_PRIO 1
>  
>  #ifdef CONFIG_RCU_BOOST
> +#include "rtmutex_common.h"
>  #define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
>  #else
>  #define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
> @@ -340,7 +341,7 @@ void rcu_read_unlock_special(struct task_struct *t)
>  	unsigned long flags;
>  	struct list_head *np;
>  #ifdef CONFIG_RCU_BOOST
> -	struct rt_mutex *rbmp = NULL;
> +	struct rt_mutex_waiter *waiter = NULL;
>  #endif /* #ifdef CONFIG_RCU_BOOST */
>  	struct rcu_node *rnp;
>  	int special;
> @@ -397,10 +398,10 @@ void rcu_read_unlock_special(struct task_struct *t)
>  #ifdef CONFIG_RCU_BOOST
>  		if (&t->rcu_node_entry == rnp->boost_tasks)
>  			rnp->boost_tasks = np;
> -		/* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
> -		if (t->rcu_boost_mutex) {
> -			rbmp = t->rcu_boost_mutex;
> -			t->rcu_boost_mutex = NULL;
> +		/* Snapshot/clear ->rcu_boost_waiter with rcu_node lock held. */
> +		if (t->rcu_boost_waiter) {
> +			waiter = t->rcu_boost_waiter;
> +			t->rcu_boost_waiter = NULL;
>  		}
>  #endif /* #ifdef CONFIG_RCU_BOOST */
>  
> @@ -426,8 +427,8 @@ void rcu_read_unlock_special(struct task_struct *t)
>  
>  #ifdef CONFIG_RCU_BOOST
>  		/* Unboost if we were boosted. */
> -		if (rbmp)
> -			rt_mutex_unlock(rbmp);
> +		if (waiter)
> +			rt_mutex_rcu_deboost_unlock(t, waiter);
>  #endif /* #ifdef CONFIG_RCU_BOOST */
>  
>  		/*
> @@ -1129,9 +1130,6 @@ void exit_rcu(void)
>  #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
>  
>  #ifdef CONFIG_RCU_BOOST
> -
> -#include "rtmutex_common.h"
> -
>  #ifdef CONFIG_RCU_TRACE
>  
>  static void rcu_initiate_boost_trace(struct rcu_node *rnp)
> @@ -1181,14 +1179,15 @@ static int rcu_boost(struct rcu_node *rnp)
>  {
>  	unsigned long flags;
>  	struct rt_mutex mtx;
> +	struct rt_mutex_waiter rcu_boost_waiter;
>  	struct task_struct *t;
>  	struct list_head *tb;
> +	int ret;
>  
>  	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
>  		return 0;  /* Nothing left to boost. */
>  
>  	raw_spin_lock_irqsave(&rnp->lock, flags);
> -
>  	/*
>  	 * Recheck under the lock: all tasks in need of boosting
>  	 * might exit their RCU read-side critical sections on their own.
> @@ -1215,7 +1214,7 @@ static int rcu_boost(struct rcu_node *rnp)
>  
>  	/*
>  	 * We boost task t by manufacturing an rt_mutex that appears to
> -	 * be held by task t.  We leave a pointer to that rt_mutex where
> +	 * be held by task t.  We leave a pointer to that rt_mutex_waiter where
>  	 * task t can find it, and task t will release the mutex when it
>  	 * exits its outermost RCU read-side critical section.  Then
>  	 * simply acquiring this artificial rt_mutex will boost task
> @@ -1230,11 +1229,30 @@ static int rcu_boost(struct rcu_node *rnp)
>  	 * section.
>  	 */
>  	t = container_of(tb, struct task_struct, rcu_node_entry);
> +	get_task_struct(t);
>  	rt_mutex_init_proxy_locked(&mtx, t);
> -	t->rcu_boost_mutex = &mtx;
>  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> -	rt_mutex_lock(&mtx);  /* Side effect: boosts task t's priority. */
> -	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */
> +
> +	debug_rt_mutex_init_waiter(&rcu_boost_waiter);
> +	/* Side effect: boosts task t's priority. */
> +	ret = rt_mutex_start_proxy_lock(&mtx, &rcu_boost_waiter, current, 0);
> +	if (WARN_ON_ONCE(ret)) {
> +		put_task_struct(t);
> +		return 0; /* temporary stop boosting */
> +	}
> +
> +	raw_spin_lock_irqsave(&rnp->lock, flags);
> +	if (&t->rcu_node_entry == rnp->exp_tasks ||
> +	    &t->rcu_node_entry == rnp->boost_tasks) {
> +		t->rcu_boost_waiter = &rcu_boost_waiter;
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +	} else {
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +		rt_mutex_rcu_deboost_unlock(t, &rcu_boost_waiter);
> +	}
> +
> +	put_task_struct(t);
> +	rt_mutex_finish_proxy_lock(&mtx, NULL, &rcu_boost_waiter, 0);
>  
>  	return ACCESS_ONCE(rnp->exp_tasks) != NULL ||
>  	       ACCESS_ONCE(rnp->boost_tasks) != NULL;
> diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
> index 0dd6aec..2f3caee 100644
> --- a/kernel/rtmutex.c
> +++ b/kernel/rtmutex.c
> @@ -734,6 +734,43 @@ rt_mutex_slowunlock(struct rt_mutex *lock)
>  	rt_mutex_adjust_prio(current);
>  }
>  
> +#ifdef CONFIG_RCU_BOOST
> +/*
> + * rt_mutex_rcu_deboost_unlock() - unlock in irq/bh/process context
> + *
> + * please revert the patch which introduces this function when
> + * rt_mutex's ->wait_lock is irq-off.

I don't think we ever want wait_lock to disable interrupts. Doing so
for just rcu boosting is not enough IMO.

Please remove that comment.

Honestly, I like this solution better than the original :-) It only
uses the pi boosting and not the rest of the rt_mutex, which is really
just overhead.

Looks good to me. Other than what I already commented:

Reviewed-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve

> + */
> +void rt_mutex_rcu_deboost_unlock(struct task_struct *owner,
> +				 struct rt_mutex_waiter *waiter)
> +{
> +	unsigned long flags;
> +	struct rt_mutex *lock = waiter->lock;
> +
> +	/*
> +	 * The correction of the following code is based on
> +	 * 1) current lock is owned by @owner
> +	 * 2) only one task(@waiter->task) is waiting on the @lock
> +	 * 3) the @waiter has been queued and keeps been queued

"keeps been queued"?  Do you mean "keeps being queued"?

> +	 */
> +	if (WARN_ON_ONCE(rt_mutex_owner(lock) != owner))
> +		return; /* 1) */
> +	if (WARN_ON_ONCE(rt_mutex_top_waiter(lock) != waiter))
> +		return; /* 2) & 3) */
> +	if (WARN_ON_ONCE(plist_node_empty(&waiter->pi_list_entry)))
> +		return; /* 2) & 3) */
> +
> +	raw_spin_lock_irqsave(&owner->pi_lock, flags);
> +	plist_del(&waiter->pi_list_entry, &owner->pi_waiters);
> +	lock->owner = NULL;
> +	raw_spin_unlock_irqrestore(&owner->pi_lock, flags);
> +
> +	wake_up_process(waiter->task);
> +	/* Undo pi boosting if necessary: */
> +	rt_mutex_adjust_prio(owner);
> +}
> +#endif /* #ifdef CONFIG_RCU_BOOST */
> +
>  /*
>   * debug aware fast / slowpath lock,trylock,unlock
>   *
> diff --git a/kernel/rtmutex_common.h b/kernel/rtmutex_common.h
> index 53a66c8..3cdbe82 100644
> --- a/kernel/rtmutex_common.h
> +++ b/kernel/rtmutex_common.h
> @@ -117,6 +117,11 @@ extern int rt_mutex_finish_proxy_lock(struct rt_mutex *lock,
>  				      struct rt_mutex_waiter *waiter,
>  				      int detect_deadlock);
>  
> +#ifdef CONFIG_RCU_BOOST
> +void rt_mutex_rcu_deboost_unlock(struct task_struct *owner,
> +				 struct rt_mutex_waiter *waiter);
> +#endif /* #ifdef CONFIG_RCU_BOOST */
> +
>  #ifdef CONFIG_DEBUG_RT_MUTEXES
>  # include "rtmutex-debug.h"
>  #else

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