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    Subject[PATCH 02/10] arm: Use clear_tasks_mm_cpumask()
    Current CPU hotplug code has some task->mm handling issues:

    1. Working with task->mm w/o getting mm or grabing the task lock is
    dangerous as ->mm might disappear (exit_mm() assigns NULL under
    task_lock(), so tasklist lock is not enough).

    We can't use get_task_mm()/mmput() pair as mmput() might sleep,
    so we must take the task lock while handle its mm.

    2. Checking for process->mm is not enough because process' main
    thread may exit or detach its mm via use_mm(), but other threads
    may still have a valid mm.

    To fix this we would need to use find_lock_task_mm(), which would
    walk up all threads and returns an appropriate task (with task
    lock held).

    clear_tasks_mm_cpumask() has all the issues fixed, so let's use it.

    Suggested-by: Oleg Nesterov <>
    Signed-off-by: Anton Vorontsov <>
    arch/arm/kernel/smp.c | 8 +-------
    1 file changed, 1 insertion(+), 7 deletions(-)

    diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
    index cdeb727..880b93a 100644
    --- a/arch/arm/kernel/smp.c
    +++ b/arch/arm/kernel/smp.c
    @@ -136,7 +136,6 @@ static void percpu_timer_stop(void);
    int __cpu_disable(void)
    unsigned int cpu = smp_processor_id();
    - struct task_struct *p;
    int ret;

    ret = platform_cpu_disable(cpu);
    @@ -166,12 +165,7 @@ int __cpu_disable(void)

    - read_lock(&tasklist_lock);
    - for_each_process(p) {
    - if (p->mm)
    - cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
    - }
    - read_unlock(&tasklist_lock);
    + clear_tasks_mm_cpumask(cpu);

    return 0;

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