Checking for task->mm 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 let's take the task lock while we care about its mm.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/arm/kernel/smp.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 57db122..85db3f2 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -168,8 +168,10 @@ int __cpu_disable(void)
 
 	read_lock(&tasklist_lock);
 	for_each_process(p) {
+		task_lock(p);
 		if (p->mm)
 			cpumask_clear_cpu(cpu, mm_cpumask(p->mm));
+		task_unlock(p);
 	}
 	read_unlock(&tasklist_lock);
 
-- 
1.7.7.6