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SubjectRe: [PATCH 5/6] _cpu_down: don't play with current->cpus_allowed
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On Monday 15 March 2010, Oleg Nesterov wrote:
> _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>

Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
> ---
>
> 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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