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

From: Anton Vorontsov
Date: Sat Mar 24 2012 - 06:29:02 EST

Many architctures clear tasks' mm_cpumask like this:

for_each_process(p) {
if (p->mm)
cpumask_clear_cpu(cpu, mm_cpumask(p->mm));

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 <anton.vorontsov@xxxxxxxxxx>
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);

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)

+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;

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