linux-kernel - Re: [PATCH RFC 5/9] RCU: CPU hotplug support for preemptible RCU

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Date:	Sun, 30 Sep 2007 20:38:49 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	mingo@...e.hu, akpm@...ux-foundation.org, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, tytso@...ibm.com, dvhltc@...ibm.com,
	tglx@...utronix.de, a.p.zijlstra@...llo.nl, bunk@...nel.org,
	ego@...ibm.com, srostedt@...hat.com
Subject: Re: [PATCH RFC 5/9] RCU: CPU hotplug support for preemptible RCU

On 09/10, Paul E. McKenney wrote:
>
> --- linux-2.6.22-d-schedclassic/kernel/rcupreempt.c	2007-08-22 15:45:28.000000000 -0700
> +++ linux-2.6.22-e-hotplugcpu/kernel/rcupreempt.c	2007-08-22 15:56:22.000000000 -0700
> @@ -125,6 +125,8 @@ enum rcu_mb_flag_values {
>  };
>  static DEFINE_PER_CPU(enum rcu_mb_flag_values, rcu_mb_flag) = rcu_mb_done;
>
> +static cpumask_t rcu_cpu_online_map = CPU_MASK_NONE;

I'd suggest to append "__read_mostly"

> +void rcu_offline_cpu_rt(int cpu)
> +{
> [...snip...]
> +	spin_lock_irqsave(&rcu_ctrlblk.fliplock, oldirq);
> +	rcu_check_mb(cpu);
> +	if (per_cpu(rcu_flip_flag, cpu) == rcu_flipped) {
> +		smp_mb();  /* Subsequent counter accesses must see new value */
> +		per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
> +		smp_mb();  /* Subsequent RCU read-side critical sections */
> +			   /*  seen -after- acknowledgement. */

Imho, all these barriers are unneeded and confusing, we can't do them on behalf
of a dead CPU anyway. Can't we just do

	per_cpu(rcu_mb_flag, cpu) = rcu_mb_done;
	per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
?

Why can't we also do

	__get_cpu_var(rcu_flipctr)[0] += per_cpu(rcu_flipctr, cpu)[0];
	per_cpu(rcu_flipctr, cpu)[0] = 0;	
	__get_cpu_var(rcu_flipctr)[1] += per_cpu(rcu_flipctr, cpu)[1];
	per_cpu(rcu_flipctr, cpu)[1] = 0;	

? This way rcu_try_flip_waitzero() can also use rcu_cpu_online_map. This cpu
is dead, nobody can modify per_cpu(rcu_flipctr, cpu). And we can't confuse
rcu_try_flip_waitzero(), we are holding rcu_ctrlblk.fliplock.

> +void __devinit rcu_online_cpu_rt(int cpu)
> +{
> +	unsigned long oldirq;
> +
> +	spin_lock_irqsave(&rcu_ctrlblk.fliplock, oldirq);
> +	cpu_set(cpu, rcu_cpu_online_map);

What if _cpu_up() fails? I think rcu_cpu_notify(CPU_UP_CANCELED) should call
rcu_offline_cpu_rt() too.

Oleg.

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