Re:Sleeping in RCU list traversal

[Jun WANG <junwang1234@gmail.com>]

Hi
>Something like this?
>
>rcu_read_lock();
>list_for_each_rcu(p, ...) {
>   ptr = list_entry(p, struct ..., list);
>   /* Grab a reference to "ptr". */
>   rcu_read_unlock();
>   my_task_that_may_sleep(ptr);
>   rcu_read_lock();
>   /* Drop a reference to "ptr". */
 >}
 >rcu_read_unlock();

>Regarding my case, memory region pointed by "ptr" never be removed.
>Do I need to grab a reference to "ptr" ?
In Document/RCU/whatisRCU.txt
Note that the value returned by rcu_dereference() is valid
	only within the enclosing RCU read-side critical section.
	For example, the following is -not- legal:

		rcu_read_lock();
		p = rcu_dereference(head.next);
		rcu_read_unlock();
		x = p->address;
		rcu_read_lock();
		y = p->data;
		rcu_read_unlock();

	Holding a reference from one RCU read-side critical section
	to another is just as illegal as holding a reference from
	one lock-based critical section to another!  Similarly,
	using a reference outside of the critical section in which
	it was acquired is just as illegal as doing so with normal
	locking.
                                                            Jun Wang