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Message-ID: <PH0PR11MB588009DA52C17BAE7388F99EDA119@PH0PR11MB5880.namprd11.prod.outlook.com>
Date: Sat, 26 Nov 2022 02:43:59 +0000
From: "Zhang, Qiang1" <qiang1.zhang@...el.com>
To: Joel Fernandes <joel@...lfernandes.org>
CC: "paulmck@...nel.org" <paulmck@...nel.org>,
"frederic@...nel.org" <frederic@...nel.org>,
"rcu@...r.kernel.org" <rcu@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug
On Fri, Nov 25, 2022 at 11:54:27PM +0800, Zqiang wrote:
> Currently, for the case of num_online_cpus() <= 1, return directly,
> indicates the end of current grace period and then release old data.
> it's not accurate, for SMP system, when num_online_cpus() is equal
> one, maybe another cpu that in offline process(after invoke
> __cpu_disable()) is still in the rude RCU-Tasks critical section
> holding the old data, this lead to memory corruption.
>
> Therefore, this commit add cpus_read_lock/unlock() before executing
> num_online_cpus().
>I am not sure if this is needed. The only way what you suggest can happen is
>if the tasks-RCU protected data is accessed after the num_online_cpus() value is
>decremented on the CPU going offline.
>
>However, the number of online CPUs value is changed on a CPU other than the
>CPU going offline.
>
>So there's no way the CPU going offline can run any code (it is already
>dead courtesy of CPUHP_AP_IDLE_DEAD). So a corruption is impossible.
>
>Or, did I miss something?
Hi joel
Suppose the system has two cpus
CPU0 CPU1
cpu_stopper_thread
take_cpu_down
__cpu_disable
dec __num_online_cpus
rcu_tasks_rude_wait_gp cpuhp_invoke_callback
num_online_cpus() == 1
return;
when __num_online_cpus == 1, the CPU1 not completely offline.
Thanks
Zqiang
>
>thanks,
>
> - Joel
>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---
> kernel/rcu/tasks.h | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 4a991311be9b..08e72c6462d8 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1033,14 +1033,30 @@ static void rcu_tasks_be_rude(struct work_struct *work)
> {
> }
>
> +static DEFINE_PER_CPU(struct work_struct, rude_work);
> +
> // Wait for one rude RCU-tasks grace period.
> static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
> {
> + int cpu;
> + struct work_struct *work;
> +
> + cpus_read_lock();
> if (num_online_cpus() <= 1)
> - return; // Fastpath for only one CPU.
> + goto end;// Fastpath for only one CPU.
>
> rtp->n_ipis += cpumask_weight(cpu_online_mask);
> - schedule_on_each_cpu(rcu_tasks_be_rude);
> + for_each_online_cpu(cpu) {
> + work = per_cpu_ptr(&rude_work, cpu);
> + INIT_WORK(work, rcu_tasks_be_rude);
> + schedule_work_on(cpu, work);
> + }
> +
> + for_each_online_cpu(cpu)
> + flush_work(per_cpu_ptr(&rude_work, cpu));
> +
> +end:
> + cpus_read_unlock();
> }
>
> void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
> --
> 2.25.1
>
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