[PATCH 2/2] oom_kill: change oom_kill.c to use for_each_thread()

[Oleg Nesterov <oleg@redhat.com>]

1. Change oom_kill.c to use for_each_thread() rather than the racy
   while_each_thread() which can loop forever if we race with exit.

   Note also that most users were buggy even if while_each_thread()
   was fine, the task can exit even _before_ rcu_read_lock().

   Fortunately the new for_each_thread() only requires the stable
   task_struct, so this change fixes both problems.

2. 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".

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/oom_kill.c |   37 ++++++++++++++++++++-----------------
 1 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1e4a600..47dd4ce 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -54,12 +54,14 @@ static DEFINE_SPINLOCK(zone_scan_lock);
  * 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;
 
-	do {
+	rcu_read_lock();
+	for_each_thread(start, tsk) {
 		if (mask) {
 			/*
 			 * If this is a mempolicy constrained oom, tsk's
@@ -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);
 		}
-	} while_each_thread(start, tsk);
+		if (ret)
+			break;
+	}
+	rcu_read_unlock();
 
-	return false;
+	return ret;
 }
 #else
 static bool has_intersects_mems_allowed(struct task_struct *tsk,
@@ -97,14 +100,14 @@ static bool has_intersects_mems_allowed(struct task_struct *tsk,
  */
 struct task_struct *find_lock_task_mm(struct task_struct *p)
 {
-	struct task_struct *t = p;
+	struct task_struct *t;
 
-	do {
+	for_each_thread(p, t) {
 		task_lock(t);
 		if (likely(t->mm))
 			return t;
 		task_unlock(t);
-	} while_each_thread(p, t);
+	}
 
 	return NULL;
 }
@@ -301,7 +304,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
 	unsigned long chosen_points = 0;
 
 	rcu_read_lock();
-	do_each_thread(g, p) {
+	for_each_process_thread(g, p) {
 		unsigned int points;
 
 		switch (oom_scan_process_thread(p, totalpages, nodemask,
@@ -323,7 +326,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
 			chosen = p;
 			chosen_points = points;
 		}
-	} while_each_thread(g, p);
+	}
 	if (chosen)
 		get_task_struct(chosen);
 	rcu_read_unlock();
@@ -406,7 +409,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 {
 	struct task_struct *victim = p;
 	struct task_struct *child;
-	struct task_struct *t = p;
+	struct task_struct *t;
 	struct mm_struct *mm;
 	unsigned int victim_points = 0;
 	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
@@ -437,7 +440,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 * still freeing memory.
 	 */
 	read_lock(&tasklist_lock);
-	do {
+	for_each_thread(p, t) {
 		list_for_each_entry(child, &t->children, sibling) {
 			unsigned int child_points;
 
@@ -455,7 +458,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 				get_task_struct(victim);
 			}
 		}
-	} while_each_thread(p, t);
+	}
 	read_unlock(&tasklist_lock);
 
 	rcu_read_lock();
-- 
1.5.5.1