[PATCH 01/10] cpu: Introduce clear_tasks_mm_cpumask() helper

Many architctures clear tasks' mm_cpumask like this:

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

The code above has several problems, such as:

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).

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.

This patch implements a small helper function that does things
correctly, i.e.:

1. We take the task's lock while whe handle its mm (we can't use
   get_task_mm()/mmput() pair as mmput() might sleep);

2. To catch exited main thread case, we use find_lock_task_mm(),
   which walks up all threads and returns an appropriate task
   (with task lock held).

Signed-off-by: Anton Vorontsov <redacted>
---
 include/linux/cpu.h |    1 +
 kernel/cpu.c        |   18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 1f65875..941e865 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h

@@ -171,6 +171,7 @@ extern void put_online_cpus(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
+void clear_tasks_mm_cpumask(int cpu);
 int cpu_down(unsigned int cpu);
 
 #ifdef CONFIG_ARCH_CPU_PROBE_RELEASE

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2060c6e..5255936 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c

@@ -10,6 +10,7 @@
 #include <linux/sched.h>
 #include <linux/unistd.h>
 #include <linux/cpu.h>
+#include <linux/oom.h>
 #include <linux/export.h>
 #include <linux/kthread.h>
 #include <linux/stop_machine.h>

@@ -171,6 +172,23 @@ void __ref unregister_cpu_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(unregister_cpu_notifier);
 
+void clear_tasks_mm_cpumask(int cpu)
+{
+	struct task_struct *p;
+
+	read_lock(&tasklist_lock);
+	for_each_process(p) {
+		struct task_struct *t;
+
+		t = find_lock_task_mm(p);
+		if (!t)
+			continue;
+		cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
+		task_unlock(t);
+	}
+	read_unlock(&tasklist_lock);
+}
+
 static inline void check_for_tasks(int cpu)
 {
 	struct task_struct *p;

-- 
1.7.9.2