Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	rcu@vger.kernel.org
Subject: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access
Date: Wed, 20 May 2020 20:28:00 +0200	[thread overview]
Message-ID: <20200520182800.sdp6t6bgbhn4kkqk@linutronix.de> (raw)
In-Reply-To: <20200520174259.GA247557@google.com>

On 2020-05-20 13:42:59 [-0400], Joel Fernandes wrote:
> Hi Sebastian,
Hi Joel,

> For pointer stability, can we just use get_local_ptr() and put_local_ptr()
> instead of adding an extra lock? This keeps the pointer stable while keeping
> the section preemptible on -rt. And we already have a lock in rcu_data, I
> prefer not to add another lock if possible.

What is this get_local_ptr() doing? I can't find it anywhere…

> I wrote a diff below with get_local_ptr() (just build tested). Does this
> solve your issue?

see below.

> > I remember Paul looked at that patch a few years ago and he said that
> > that disabling interrupts here is important and matches the other part
> > instance where the interrupts are disabled. Looking at it now, it seems
> > that there is just pointer stability but I can't tell if
> > rcu_segcblist_pend_cbs() needs more than just this.
> 
> Which 'other part' are you referring to? Your patch removed local_irq_save()
> from other places as well right?

The patch converted hunks.

> thanks,
> 
>  - Joel
> 
> ---8<-----------------------
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 8ff71e5d0fe8b..5f49919205317 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -778,13 +778,17 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
>  	unsigned long tlast;
>  
>  	/* If the local srcu_data structure has callbacks, not idle.  */
> -	local_irq_save(flags);
> -	sdp = this_cpu_ptr(ssp->sda);
> +	sdp = get_local_ptr(ssp->sda);
> +	spin_lock_irqsave_rcu_node(sdp, flags);

You acquire the node lock which was not acquired before. Is that okay?
How is get_local_ptr() different to raw_cpu_ptr()?

>  	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
> -		local_irq_restore(flags);
> +		spin_unlock_irqrestore_rcu_node(sdp, flags);
> +		put_local_ptr(sdp);
>  		return false; /* Callbacks already present, so not idle. */
>  	}
> -	local_irq_restore(flags);
> +
> +	spin_unlock_irqrestore_rcu_node(sdp, flags);
> +	put_local_ptr(sdp);
>  
>  	/*
>  	 * No local callbacks, so probabalistically probe global state.

Sebastian