sched: silence PROVE_RCU in sched_fork()

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.

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

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index f8b8996228dd..a2d215d132f6 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2494,7 +2494,16 @@ 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()))