rcu: apply RCU protection to wake_affine()

The task_group() function returns a pointer that must be protected
by either RCU, the ->alloc_lock, or the cgroup lock (see the
rcu_dereference_check() in task_subsys_state(), which is invoked by
task_group()).  The wake_affine() function currently does none of these,
which means that a concurrent update would be within its rights to free
the structure returned by task_group().  Because wake_affine() uses this
structure only to compute load-balancing heuristics, there is no reason
to acquire either of the two locks.

Therefore, this commit introduces an RCU read-side critical section that
starts before the first call to task_group() and ends after the last use
of the "tg" pointer returned from task_group().  Thanks to Li Zefan for
pointing out the need to extend the RCU read-side critical section from
that proposed by the original patch.

Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index eed35eded602..a878b5332daa 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1240,6 +1240,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
          * effect of the currently running task from the load
          * of the current CPU:
          */
+        rcu_read_lock();
         if (sync) {
                 tg = task_group(current);
                 weight = current->se.load.weight;
@@ -1275,6 +1276,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
                 balanced = this_eff_load <= prev_eff_load;
         } else
                 balanced = true;
+        rcu_read_unlock();
 
         /*
          * If the currently running task will sleep within