Re: while_each_thread() under rcu_read_lock() is broken?

[Oleg Nesterov <oleg@redhat.com>]

On 06/21, Oleg Nesterov wrote:
>
> So, I am thinking about the first attempt
>
> 	#define while_each_thread(g, t) \
> 		while ((t = next_thread(t)) != g && pid_alive(g))
>
> again. But this means while_each_thread() can miss more threads
> than it currently can under the same conditions. Correct, but
> not good.

Not good, but correct ;) Probably it makes sense to fix the problem
anyway, then think about the more optimal fix.

	static inline struct task_struct *
	next_thread_careful(const struct task_struct *g, const struct task_struct *t)
	{
		t = next_thread(t);
		/*
		 * this pairs with the implicit barrier between detach_pid()
		 * and list_del_rcu(g->thread_group) in __unhash_process(g).
		 */
		smp_rmb();
		if (likely(pid_alive(g)))
			return t;
		else
			return g;
	}

	#define while_each_thread(g, t) \
		while ((t = next_thread_careful(t)) != g)

I think this should work. detach_pid() does unlock + lock at least
once and thus we have the barrier (this worth a comment or we
can add the explicit wmb() in __unhash_process).

Paul, Roland, do you see any problems from the correctness pov,
or a better fix for now?

Perhaps it also makes sense to keep the old variant renamed to
while_each_thread_locked(), I dunno.

Oleg.