[PATCH 2/4] exit: Use rcu instead of get_task_struct() in mm_update_next_owner()

[Kirill Tkhai]

Since release_task() puts final task_struct::usage counter
at least one rcu grace period after removing from task list:

        __exit_signal()
          __unhash_process()
        call_rcu(&p->rcu, delayed_put_task_struct)

rcu_read_lock() guarantees nobody release task_struct memory.
So, it's possible to use this primitive instead of get_task_struct().

Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
---
 kernel/exit.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 9fb7b699bdeb..5c42a9b9c1d7 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -464,7 +464,7 @@ void mm_update_next_owner(struct mm_struct *mm)
        return;
 
 assign_new_owner:
-       get_task_struct(c);
+       rcu_read_lock();
        read_unlock(&tasklist_lock);
        BUG_ON(c == p);
 
@@ -475,12 +475,12 @@ void mm_update_next_owner(struct mm_struct *mm)
        task_lock(c);
        if (c->mm != mm) {
                task_unlock(c);
-               put_task_struct(c);
+               rcu_read_unlock();
                goto retry;
        }
        mm->owner = c;
        task_unlock(c);
-       put_task_struct(c);
+       rcu_read_unlock();
 }
 #endif /* CONFIG_MEMCG */