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

Oleg Nesterov
2007-09-30T16:34:36Z

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.