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

[Linus Torvalds]

On Wed, Sep 4, 2019 at 9:33 AM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> I thought the point of these rcu_users was to be able to do:
>
> rcu_read_lock()
> p = rcu_dereference(task)
> if (!refcount_inc_not_zero(p->rcu_users)) {

No. Because of the shared state, you can't do that from RCU context.

But you *could* increase rcu_users from within the process context
itself (as long as you do it before the exit path, ie during normal
system call execution), or possibly while holding the tasklist_lock
and verifying that the task hasn't died yet.

I'm not sure there is any sensible case for doing that, though. It
would have to have a similar pattern to the runqueue use, where you
add a new RCU lookup point for the task. I'm sure something like that
_could_ exist, I just can't think of any right now.

             Linus