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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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