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

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Message-ID: <20190904153245.GF24568@redhat.com>
Date:   Wed, 4 Sep 2019 17:32:46 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 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.