rcu: Stop expedited grace periods from relying on stop-machine
The CPU-selection code in sync_rcu_exp_select_cpus() disables preemption to prevent the cpu_online_mask from changing. However, this relies on the stop-machine mechanism in the CPU-hotplug offline code, which is not desirable (it would be good to someday remove the stop-machine mechanism). This commit therefore instead uses the relevant leaf rcu_node structure's ->ffmask, which has a bit set for all CPUs that are fully functional. A given CPU's bit is cleared very early during offline processing by rcutree_offline_cpu() and set very late during online processing by rcutree_online_cpu(). Therefore, if a CPU's bit is set in this mask, and preemption is disabled, we have to be before the synchronize_sched() in the CPU-hotplug offline code, which means that the CPU is guaranteed to be workqueue-ready throughout the duration of the enclosing preempt_disable() region of code. This also has the side-effect of using WORK_CPU_UNBOUND if all the CPUs for this leaf rcu_node structure are offline, which is an acceptable difference in behavior. Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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651022382c
Коммит
9cac83a57e
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@ -450,10 +450,12 @@ static void sync_rcu_exp_select_cpus(smp_call_func_t func)
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}
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INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
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preempt_disable();
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cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
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cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
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/* If all offline, queue the work on an unbound CPU. */
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if (unlikely(cpu > rnp->grphi))
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if (unlikely(cpu > rnp->grphi - rnp->grplo))
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cpu = WORK_CPU_UNBOUND;
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else
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cpu += rnp->grplo;
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queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
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preempt_enable();
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rnp->exp_need_flush = true;
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