Re: [PATCH RFC 5/9] RCU: CPU hotplug support for preemptible RCU
From: Oleg Nesterov <hidden>
Date: 2007-09-30 16:34:36
Also in:
lkml
On 09/10, Paul E. McKenney wrote:
quoted hunk ↗ jump to hunk
--- 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.