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From: Li Zefan on 23 Jun 2010 03:30
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 <a.p.zijlstra(a)chello.nl>

For this patch:

Reviewed-by: Li Zefan <lizf(a)cn.fujitsu.com>

> ---
> 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()))
>
>
>
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