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    Subject[PATCH 1/4] sched: consult online mask instead of active in select_fallback_rq()
    If called after sched_class chooses a CPU which isn't in a task's
    cpus_allowed mask, select_fallback_rq() can end up migrating a task
    which is bound to an !active but online cpu to an active cpu. This is
    dangerous because active is cleared before CPU_DOWN_PREPARE is called
    and subsystems expect affinities of kthreads and other tasks to be
    maintained till their CPU_DOWN_PREPARE callbacks are complete.

    Consult cpu_online_mask instead.

    This problem is triggered by cmwq. During CPU_DOWN_PREPARE, hotplug
    callback creates the trustee kthread and kthread_bind()s it to the
    target cpu, and the trustee is expected to run on that cpu.

    Signed-off-by: Tejun Heo <>
    Cc: Peter Zijlstra <>
    Cc: Ingo Molnar <>
    kernel/sched.c | 4 ++--
    1 files changed, 2 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched.c b/kernel/sched.c
    index b531d79..aca4a20 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -2262,12 +2262,12 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
    const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(cpu));

    /* Look for allowed, online CPU in same node. */
    - for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
    + for_each_cpu_and(dest_cpu, nodemask, cpu_online_mask)
    if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
    return dest_cpu;

    /* Any allowed, online CPU? */
    - dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
    + dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_online_mask);
    if (dest_cpu < nr_cpu_ids)
    return dest_cpu;


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