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 <[email protected]> Signed-off-by: Oleg Nesterov <[email protected]> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
