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Subject[104/123] sched: apply RCU protection to wake_affine()
From: Daniel J Blueman <>

commit f3b577dec1f2ce32d2db6d2ca6badff7002512af upstream

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 <>
Signed-off-by: Paul E. McKenney <>
Signed-off-by: Mike Galbraith <>
Signed-off-by: Greg Kroah-Hartman <>
kernel/sched_fair.c | 2 ++
1 file changed, 2 insertions(+)

--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1250,6 +1250,7 @@ static int wake_affine(struct sched_doma
* 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_doma
balanced = !this_load ||
100*(this_load + effective_load(tg, this_cpu, weight, weight)) <=
imbalance*(load + effective_load(tg, prev_cpu, 0, weight));
+ rcu_read_unlock();

* If the currently running task will sleep within

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