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

[Oleg Nesterov <oleg@redhat.com>]

On 06/21, Oleg Nesterov wrote:
>
> 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.

Well. but current_is_single_threaded() and zap_threads() have to
use next_thread() or while_each_thread_locked() in this case...

Oleg.