Re: [PATCH] rcu: revert "Allow post-unlock reference for rt_mutex" to avoid priority-inversion

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

On Tue, Nov 18, 2014 at 04:30:01PM +0800, Lai Jiangshan wrote:
> The patch dfeb9765ce3c ("Allow post-unlock reference for rt_mutex")
> ensured rcu-boost safe even the rt_mutex has post-unlock reference.
> 
> But rt_mutex allowing post-unlock reference is definitely a bug and it was
> fixed by the commit 27e35715df54 ("rtmutex: Plug slow unlock race").
> This fix made the previous patch (dfeb9765ce3c) useless.
> 
> And even worse, the priority-inversion introduced by the the previous
> patch still exists.
> 
> rcu_read_unlock_special() {
> 	rt_mutex_unlock(&rnp->boost_mtx);
> 	/* Priority-Inversion:
> 	 * the current task had been deboosted and preempted as a low
> 	 * priority task immediately, it could wait long before reschedule in,
> 	 * and the rcu-booster also waits on this low priority task and sleeps.
> 	 * This priority-inversion makes rcu-booster can't work
> 	 * as expected.
> 	 */
> 	complete(&rnp->boost_completion);
> }
> 
> Just revert the patch to avoid it.
> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>

Good catch, I had indeed forgotten this one.  Queued for 3.20, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/tree.h        |    5 -----
>  kernel/rcu/tree_plugin.h |    8 +-------
>  2 files changed, 1 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 49b3da7..f14580c 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -172,11 +172,6 @@ struct rcu_node {
>  				/*  queued on this rcu_node structure that */
>  				/*  are blocking the current grace period, */
>  				/*  there can be no such task. */
> -	struct completion boost_completion;
> -				/* Used to ensure that the rt_mutex used */
> -				/*  to carry out the boosting is fully */
> -				/*  released with no future boostee accesses */
> -				/*  before that rt_mutex is re-initialized. */
>  	struct rt_mutex boost_mtx;
>  				/* Used only for the priority-boosting */
>  				/*  side effect, not as a lock. */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 152f0e3..272d837 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -427,10 +427,8 @@ void rcu_read_unlock_special(struct task_struct *t)
> 
>  #ifdef CONFIG_RCU_BOOST
>  		/* Unboost if we were boosted. */
> -		if (drop_boost_mutex) {
> +		if (drop_boost_mutex)
>  			rt_mutex_unlock(&rnp->boost_mtx);
> -			complete(&rnp->boost_completion);
> -		}
>  #endif /* #ifdef CONFIG_RCU_BOOST */
> 
>  		/*
> @@ -1100,15 +1098,11 @@ static int rcu_boost(struct rcu_node *rnp)
>  	 */
>  	t = container_of(tb, struct task_struct, rcu_node_entry);
>  	rt_mutex_init_proxy_locked(&rnp->boost_mtx, t);
> -	init_completion(&rnp->boost_completion);
>  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  	/* Lock only for side effect: boosts task t's priority. */
>  	rt_mutex_lock(&rnp->boost_mtx);
>  	rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
> 
> -	/* Wait for boostee to be done w/boost_mtx before reinitializing. */
> -	wait_for_completion(&rnp->boost_completion);
> -
>  	return ACCESS_ONCE(rnp->exp_tasks) != NULL ||
>  	       ACCESS_ONCE(rnp->boost_tasks) != NULL;
>  }
> -- 
> 1.7.4.4
>