linux-kernel - Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access

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Date:	Mon, 29 Mar 2010 15:43:43 -0700
From:	Paul Menage <menage@...gle.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, abogani@...ware.it
Subject: Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access

On Mon, Mar 29, 2010 at 2:15 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> Add an rcu_read_lock() / rcu_read_unlock() pair to protect a fork-time
> cgroup access.  This seems likely to be a false positive.
>
> Located by: Alessio Igor Bogani <abogani@...ware.it>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>
>  sched.c |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9ab3cd7..d4bb5e0 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2621,7 +2621,9 @@ void sched_fork(struct task_struct *p, int clone_flags)
>        if (p->sched_class->task_fork)
>                p->sched_class->task_fork(p);
>
> +       rcu_read_lock();
>        set_task_cpu(p, cpu);
> +       rcu_read_unlock();

I think you're right that this is a false positive - it would only be
a problem if it were possible for the task to be moved to a different
cgroup, and I think that shouldn't be the case at this point in the
fork path since the new process isn't visible on the tasklist yet,
right?

Paul
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