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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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