lkml.org 
[lkml]   [2022]   [Nov]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRE: [PATCH] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug
    Date
    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@intel.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
    >

    \
     
     \ /
      Last update: 2022-11-26 03:45    [W:2.282 / U:0.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site