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Message-ID: <Y4GXUFn12oVkk/d8@google.com> Date: Sat, 26 Nov 2022 04:34:24 +0000 From: Joel Fernandes <joel@...lfernandes.org> To: "Zhang, Qiang1" <qiang1.zhang@...el.com> Cc: "paulmck@...nel.org" <paulmck@...nel.org>, "frederic@...nel.org" <frederic@...nel.org>, neeraj.iitr10@...il.com, "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 Sat, Nov 26, 2022 at 02:43:59AM +0000, Zhang, Qiang1 wrote: > 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 Thanks for clarifying! You are right, this can be a problem for anything in the stop machine on the CPU going offline from CPUHP_AP_ONLINE to CPUHP_AP_IDLE_DEAD, during which the code execute on that CPU is not accounted for in num_online_cpus(). Actually Neeraj found a similar issue 2 years ago and instead of hotplug lock, he added a new attribute to rcu_state to track number of CPUs. See: https://lore.kernel.org/r/20200923210313.GS29330@paulmck-ThinkPad-P72 https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2317853.html Could we do something similar? Off note is the comment in that thread: Actually blocking CPU hotplug would not only result in excessive overhead, but would also unnecessarily impede CPU-hotplug operations. Neeraj is also on the thread and could chime in. Thanks, - Joel > 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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