Subject: Re: [PATCH] sched: silence PROVE_RCU in sched_fork()

From: Li Zefan <[email-masked]>
Date: Wed, 23 Jun 2010 15:25:50 +0800

Peter Zijlstra wrote:
> Paul Menage, Li Zefan, any comments?
> 
> 
> ---
> Because cgroup_fork() is ran before sched_fork() [ from copy_process() ]
> and the child's pid is not yet visible the child is pinned to its
> cgroup. Therefore we can silence this warning.
> 

The explanation is correct.

We silenced another warning according to the same reason.

See freezer_fork() in kernel/cgroup_freezer.c

> A nicer solution would be moving cgroup_fork() to right after
> dup_task_struct() and exclude PF_STARTING from task_subsys_state().
> 

Seems PF_STARTING is set in copy_flags(), that's after dup_task_struct() and
before cgroup_fork(). But it's cleared after copy_process(), that's after
the task is linked into tasklist, so this doesn't seem work...

> Signed-off-by: Peter Zijlstra <[email-masked]>

For this patch:

Reviewed-by: Li Zefan <[email-masked]>

> ---
>  kernel/sched.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index b697606..2e79518 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2561,7 +2561,16 @@ void sched_fork(struct task_struct *p, int clone_flags)
>  	if (p->sched_class->task_fork)
>  		p->sched_class->task_fork(p);
>  
> +	/*
> +	 * The child is not yet in the pid-hash so no cgroup attach races,
> +	 * and the cgroup is pinned to this child due to cgroup_fork()
> +	 * is ran before sched_fork().
> +	 *
> +	 * Silence PROVE_RCU.
> +	 */
> +	rcu_read_lock();
>  	set_task_cpu(p, cpu);
> +	rcu_read_unlock();
>  
>  #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
>  	if (likely(sched_info_on()))
> 
> 
>