Messages in this thread | | | Date | Sun, 30 Sep 2007 20:38:49 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH RFC 5/9] RCU: CPU hotplug support for preemptible RCU |
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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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