Re: [PATCH 1/3] task: Add a count of task rcu users

[Oleg Nesterov]

On 09/04, Frederic Weisbecker wrote:
>
> So what happens if, say:
>
>
>            CPU 1                         CPU 2
>    --------------------------------------------------------------
>    rcu_read_lock()
>    p = rcu_dereference(rq->task)
>    if (refcount_inc_not_zero(p->rcu_users)) {
>        .....
>                                          release_task() {
>                                              put_task_struct_rcu_user() {
>                                                  call_rcu() {
>                                                      queue rcu_head

in this particular case call_rcu() won't be called, so

>                                                  }
>                                              }
>                                          }
>        put_task_struct_rcu_user(); //here rcu_users has been overwritten

rcu_users won't be overwritten.

But nobody should try to increment ->rcu_users,

	rcu_read_lock();
	p = rcu_dereference(rq->task);
	refcount_inc_not_zero(p->rcu_users);

is already wrong because both release_task/last-schedule can happen in
between, before refcount_inc_not_zero().

Oleg.