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

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Date:   Wed, 4 Sep 2019 16:44:16 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.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 Mon, Sep 02, 2019 at 11:51:34PM -0500, Eric W. Biederman wrote:
> 
> Add a count of the number of rcu users (currently 1) of the task
> struct so that we can later add the scheduler case and get rid of the
> very subtle task_rcu_dereference, and just use rcu_dereference.
> 
> As suggested by Oleg have the count overlap rcu_head so that no
> additional space in task_struct is required.
> 
> Inspired-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Inspired-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  include/linux/sched.h      | 5 ++++-
>  include/linux/sched/task.h | 1 +
>  kernel/exit.c              | 7 ++++++-
>  kernel/fork.c              | 7 +++----
>  4 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 9f51932bd543..99a4518b9b17 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1142,7 +1142,10 @@ struct task_struct {
>  
>  	struct tlbflush_unmap_batch	tlb_ubc;
>  
> -	struct rcu_head			rcu;
> +	union {
> +		refcount_t		rcu_users;
> +		struct rcu_head		rcu;

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


Thanks.