Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

[Oleg Nesterov <oleg@redhat.com>]

On 10/20, Oleg Nesterov wrote:
>
> Again, perhaps we will need to change the lifetime rules for task_struct
> anyway, if we have more problems like this. But until then this looks like
> an overkill to me. Plus rq_curr_if_not_put() looks too subtle, and it is
> not generic.

Yes... otoh, perhaps we can do something more generic? Something like

	struct task_struct *xxx(struct task_struct **ptask)
	{
		struct task_struct *task;
		void *sighand;
	retry:
		task = ACCESS_ONCE(*ptask);
		if (!task)
			return NULL;

		if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {
			if (probe_kernel_read(&sighand, &task->sighand, sizeof(sighand)))
				goto retry;
		} else {
			sighand = task->sighand;
		}

		if (!sighand)
			return NULL;
		/*
		 * Pairs with atomic_dec_and_test() in put_task_struct(task).
		 * If we have read the freed/reused memory, we must see that
		 * the pointer was updated.
		 */
		smp_rmb();
		if (task != ACCESS_ONCE(*ptask))
			goto retry;

		return task;
	}

task_numa_compare() can do cur = xxx(&rc->curr), but this helper can work
with any "task_struct *" pointer assuming that somehow this pointer is
cleared or changed before the final put_task_struct().

What do you think? Peter?

Oleg.