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Subject[PATCH 5/6] _cpu_down: don't play with current->cpus_allowed
_cpu_down() changes the current task's affinity and then recovers it at
the end. The problems are well known: we can't restore old_allowed if it
was bound to the now-dead-cpu, and we can race with the userspace which
can change cpu-affinity during unplug.

_cpu_down() should not play with current->cpus_allowed at all. Instead,
take_cpu_down() can migrate the caller of _cpu_down() after __cpu_disable()
removes the dying cpu from cpu_online_mask.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
include/linux/sched.h | 1 +
kernel/sched.c | 2 +-
kernel/cpu.c | 18 ++++++------------
3 files changed, 8 insertions(+), 13 deletions(-)
--- 34-rc1/include/linux/sched.h~4_CPU_DOWN_AFFINITY 2010-03-15 09:37:46.000000000 +0100
+++ 34-rc1/include/linux/sched.h 2010-03-15 09:41:51.000000000 +0100
@@ -1843,6 +1843,7 @@ extern void sched_clock_idle_sleep_event
extern void sched_clock_idle_wakeup_event(u64 delta_ns);

#ifdef CONFIG_HOTPLUG_CPU
+extern void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p);
extern void idle_task_exit(void);
#else
static inline void idle_task_exit(void) {}
--- 34-rc1/kernel/sched.c~4_CPU_DOWN_AFFINITY 2010-03-15 09:41:28.000000000 +0100
+++ 34-rc1/kernel/sched.c 2010-03-15 09:41:51.000000000 +0100
@@ -5503,7 +5503,7 @@ static int migration_thread(void *data)
/*
* Figure out where task on dead CPU should go, use force if necessary.
*/
-static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
+void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
{
struct rq *rq = cpu_rq(dead_cpu);
int needs_cpu, dest_cpu;
--- 34-rc1/kernel/cpu.c~4_CPU_DOWN_AFFINITY 2010-03-15 09:37:46.000000000 +0100
+++ 34-rc1/kernel/cpu.c 2010-03-15 09:41:51.000000000 +0100
@@ -163,6 +163,7 @@ static inline void check_for_tasks(int c
}

struct take_cpu_down_param {
+ struct task_struct *caller;
unsigned long mod;
void *hcpu;
};
@@ -171,6 +172,7 @@ struct take_cpu_down_param {
static int __ref take_cpu_down(void *_param)
{
struct take_cpu_down_param *param = _param;
+ unsigned int cpu = (unsigned long)param->hcpu;
int err;

/* Ensure this CPU doesn't handle any more interrupts. */
@@ -181,6 +183,8 @@ static int __ref take_cpu_down(void *_pa
raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
param->hcpu);

+ if (task_cpu(param->caller) == cpu)
+ move_task_off_dead_cpu(cpu, param->caller);
/* Force idle task to run as soon as we yield: it should
immediately notice cpu is offline and die quickly. */
sched_idle_next();
@@ -191,10 +195,10 @@ static int __ref take_cpu_down(void *_pa
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
int err, nr_calls = 0;
- cpumask_var_t old_allowed;
void *hcpu = (void *)(long)cpu;
unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
struct take_cpu_down_param tcd_param = {
+ .caller = current,
.mod = mod,
.hcpu = hcpu,
};
@@ -205,9 +209,6 @@ static int __ref _cpu_down(unsigned int
if (!cpu_online(cpu))
return -EINVAL;

- if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
- return -ENOMEM;
-
cpu_hotplug_begin();
set_cpu_active(cpu, false);
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
@@ -224,10 +225,6 @@ static int __ref _cpu_down(unsigned int
goto out_release;
}

- /* Ensure that we are not runnable on dying cpu */
- cpumask_copy(old_allowed, &current->cpus_allowed);
- set_cpus_allowed_ptr(current, cpu_active_mask);
-
err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
if (err) {
set_cpu_active(cpu, true);
@@ -236,7 +233,7 @@ static int __ref _cpu_down(unsigned int
hcpu) == NOTIFY_BAD)
BUG();

- goto out_allowed;
+ goto out_release;
}
BUG_ON(cpu_online(cpu));

@@ -254,8 +251,6 @@ static int __ref _cpu_down(unsigned int

check_for_tasks(cpu);

-out_allowed:
- set_cpus_allowed_ptr(current, old_allowed);
out_release:
cpu_hotplug_done();
if (!err) {
@@ -263,7 +258,6 @@ out_release:
hcpu) == NOTIFY_BAD)
BUG();
}
- free_cpumask_var(old_allowed);
return err;
}



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