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:
>
> 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?

And if we introduce this helper, it would better to check "sighand != NULL"
after "task != *ptask":

	struct task_struct *xxx(struct task_struct **ptask)
	{
		struct task_struct *task;
		struct sighand_struct *sighand;

	retry:
		task = ACCESS_ONCE(*ptask);
		if (!task)
			return task;

		if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {
			if (probe_kernel_read(&sighand, &task->sighand, sizeof(sighand)))
				goto retry;
		} else {
			sighand = task->sighand;
		}
		/*
		 * 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 (unlikely(task != ACCESS_ONCE(*ptask)))
			goto retry;
		/*
		 * release_task(task) was already called, potentially before
		 * the caller took rcu_read_lock() and in this case it can be
		 * freed before rcu_read_unlock().
		 */
		if (!sighand)
			return NULL;
		return task;
	}

Of course, task_numa_compare() do not really need "retry", and task == NULL
is not possible. But this way the new helper can (probably) have more users,
and this just looks better imo.

Oleg.