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Subject[PATCH 2/4] exit: Use rcu instead of get_task_struct() in mm_update_next_owner()
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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 <ktkhai@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 */

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