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    Subject[PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign()
    The lockless get_task_struct(tsk) is only safe if tsk == current
    and didn't pass exit_notify(), or if this tsk was found on a rcu
    protected list (say, for_each_process() or find_task_by_vpid()).
    IOW, it is only safe if release_task() was not called before we
    take rcu_read_lock(), in this case we can rely on the fact that
    delayed_put_pid() can not drop the (potentially) last reference
    until rcu_read_unlock().

    And as Kirill pointed out task_numa_compare()->task_numa_assign()
    path does get_task_struct(dst_rq->curr) and this is not safe. The
    task_struct itself can't go away, but rcu_read_lock() can't save
    us from the final put_task_struct() in finish_task_switch(); this
    reference goes away without rcu gp.

    Reported-by: Kirill Tkhai <ktkhai@parallels.com>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---
    kernel/sched/fair.c | 8 +++++++-
    1 files changed, 7 insertions(+), 1 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 0090e8c..52049b9 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -1158,7 +1158,13 @@ static void task_numa_compare(struct task_numa_env *env,

    rcu_read_lock();
    cur = ACCESS_ONCE(dst_rq->curr);
    - if (cur->pid == 0) /* idle */
    + /*
    + * No need to move the exiting task, and this ensures that ->curr
    + * wasn't reaped and thus get_task_struct() in task_numa_assign()
    + * is safe; note that rcu_read_lock() can't protect from the final
    + * put_task_struct() after the last schedule().
    + */
    + if (is_idle_task(cur) || (cur->flags & PF_EXITING))
    cur = NULL;

    /*
    --
    1.5.5.1



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