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

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Message-ID: <20190905145939.GB18251@lenoir>
Date:   Thu, 5 Sep 2019 16:59:40 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 1/3] task: Add a count of task rcu users

On Wed, Sep 04, 2019 at 11:20:03AM -0700, Linus Torvalds wrote:
> On Wed, Sep 4, 2019 at 9:33 AM Frederic Weisbecker <frederic@...nel.org> 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.

Yeah indeed, in fact I just hadn't read the patches entirely so I missed
the point. I see now that the purpose of rcu_users is to postpone the RCU
delayed put_task_struct() at least once we are done with both final schedule
out and release_task().

Sorry for the noise :o)