Re: [PATCH 06/11] rcu: clear t->rcu_read_unlock_special in one go

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Lai Jiangshan <laijs@linux.alibaba.com>
Cc: linux-kernel@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	rcu@vger.kernel.org
Subject: Re: [PATCH 06/11] rcu: clear t->rcu_read_unlock_special in one go
Date: Fri, 1 Nov 2019 09:58:46 -0700	[thread overview]
Message-ID: <20191101165846.GA21445@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20191101121056.GB17910@paulmck-ThinkPad-P72>

On Fri, Nov 01, 2019 at 05:10:56AM -0700, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:08:01AM +0000, Lai Jiangshan wrote:
> > Clearing t->rcu_read_unlock_special in one go makes the code
> > more clearly.
> > 
> > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> Nice simplification!  I had to hand-apply it due to not having taken the
> earlier patches, plus I redid the commit log.  Could you please check
> the version shown below?

Except that this simplification depends on having moved the check of
(!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) early, and
thus results in rcutorture failures due to tasks failing to be dequeued.

From what I can see, the only early exit that matters is the first one,
so I am simply removing those within the "if" statements.

							Thanx, Paul

> ------------------------------------------------------------------------
> 
> commit 0bef7971edbbd35ed4d1682a465f682077981e85
> Author: Lai Jiangshan <laijs@linux.alibaba.com>
> Date:   Fri Nov 1 05:06:21 2019 -0700
> 
>     rcu: Clear ->rcu_read_unlock_special only once
>     
>     In rcu_preempt_deferred_qs_irqrestore(), ->rcu_read_unlock_special is
>     cleared one piece at a time.  Given that the "if" statements in this
>     function use the copy in "special", this commit removes the clearing
>     of the individual pieces in favor of clearing ->rcu_read_unlock_special
>     in one go just after it has been determined to be non-zero.
>     
>     Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 8d0e8c1..d113923 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -444,11 +444,9 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>  		local_irq_restore(flags);
>  		return;
>  	}
> -	t->rcu_read_unlock_special.b.exp_hint = false;
> -	t->rcu_read_unlock_special.b.deferred_qs = false;
> +	t->rcu_read_unlock_special.s = 0;
>  	if (special.b.need_qs) {
>  		rcu_qs();
> -		t->rcu_read_unlock_special.b.need_qs = false;
>  		if (!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) {
>  			local_irq_restore(flags);
>  			return;
> @@ -471,7 +469,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>  
>  	/* Clean up if blocked during RCU read-side critical section. */
>  	if (special.b.blocked) {
> -		t->rcu_read_unlock_special.b.blocked = false;
>  
>  		/*
>  		 * Remove this task from the list it blocked on.  The task