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

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Message-ID: <20200520102407.GF317569@hirez.programming.kicks-ass.net>
Date:   Wed, 20 May 2020 12:24:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org
Subject: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data
 access

On Tue, May 19, 2020 at 10:19:07PM +0200, Sebastian Andrzej Siewior wrote:

> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 0c71505f0e19c..8d2b5f75145d7 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -25,6 +25,7 @@
>  #include <linux/delay.h>
>  #include <linux/module.h>
>  #include <linux/srcu.h>
> +#include <linux/locallock.h>
>  
>  #include "rcu.h"
>  #include "rcu_segcblist.h"
> @@ -735,6 +736,7 @@ static void srcu_flip(struct srcu_struct *ssp)
>  	smp_mb(); /* D */  /* Pairs with C. */
>  }
>  
> +static DEFINE_LOCAL_LOCK(sda_lock);
>  /*
>   * If SRCU is likely idle, return true, otherwise return false.
>   *
> @@ -765,13 +767,13 @@ 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);
> +	local_lock_irqsave(sda_lock, flags);
>  	sdp = this_cpu_ptr(ssp->sda);
>  	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
> -		local_irq_restore(flags);
> +		local_unlock_irqrestore(sda_lock, flags);
>  		return false; /* Callbacks already present, so not idle. */
>  	}
> -	local_irq_restore(flags);
> +	local_unlock_irqrestore(sda_lock, flags);

Would it perhaps make sense to stick the local_lock in struct srcu_data ?