[PATCH v2 3/4] oom_kill: has_intersects_mems_allowed() needs rcu_read_lock()

[Oleg Nesterov <oleg@redhat.com>]

At least out_of_memory() calls has_intersects_mems_allowed()
without even rcu_read_lock(), this is obviously buggy.

Add the necessary rcu_read_lock(). This means that we can not
simply return from the loop, we need "bool ret" and "break".

While at it, swap the names of task_struct's (the argument and
the local). This cleanups the code a little bit and avoids the
unnecessary initialization.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-and-Tested-by: Sergey Dyasly <dserrg@gmail.com>
Reviewed-by: Sameer Nanda <snanda@chromium.org>
---
 mm/oom_kill.c |   19 +++++++++++--------
 1 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 96d7945..0d8ad1e 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -47,18 +47,20 @@ static DEFINE_SPINLOCK(zone_scan_lock);
 #ifdef CONFIG_NUMA
 /**
  * has_intersects_mems_allowed() - check task eligiblity for kill
- * @tsk: task struct of which task to consider
+ * @start: task struct of which task to consider
  * @mask: nodemask passed to page allocator for mempolicy ooms
  *
  * Task eligibility is determined by whether or not a candidate task, @tsk,
  * shares the same mempolicy nodes as current if it is bound by such a policy
  * and whether or not it has the same set of allowed cpuset nodes.
  */
-static bool has_intersects_mems_allowed(struct task_struct *tsk,
+static bool has_intersects_mems_allowed(struct task_struct *start,
 					const nodemask_t *mask)
 {
-	struct task_struct *start = tsk;
+	struct task_struct *tsk;
+	bool ret = false;
 
+	rcu_read_lock();
 	for_each_thread(start, tsk) {
 		if (mask) {
 			/*
@@ -67,19 +69,20 @@ static bool has_intersects_mems_allowed(struct task_struct *tsk,
 			 * mempolicy intersects current, otherwise it may be
 			 * needlessly killed.
 			 */
-			if (mempolicy_nodemask_intersects(tsk, mask))
-				return true;
+			ret = mempolicy_nodemask_intersects(tsk, mask);
 		} else {
 			/*
 			 * This is not a mempolicy constrained oom, so only
 			 * check the mems of tsk's cpuset.
 			 */
-			if (cpuset_mems_allowed_intersects(current, tsk))
-				return true;
+			ret = cpuset_mems_allowed_intersects(current, tsk);
 		}
+		if (ret)
+			break;
 	}
+	rcu_read_unlock();
 
-	return false;
+	return ret;
 }
 #else
 static bool has_intersects_mems_allowed(struct task_struct *tsk,
-- 
1.5.5.1