[SRU Trusty][PATCH 3/3] oom_kill: add rcu_read_lock() into find_lock_task_mm()

[Kamal Mostafa]

From: Oleg Nesterov <oleg at redhat.com>

BugLink: http://bugs.launchpad.net/bugs/1592429

find_lock_task_mm() expects it is called under rcu or tasklist lock, but
it seems that at least oom_unkillable_task()->task_in_mem_cgroup() and
mem_cgroup_out_of_memory()->oom_badness() can call it lockless.

Perhaps we could fix the callers, but this patch simply adds rcu lock
into find_lock_task_mm().  This also allows to simplify a bit one of its
callers, oom_kill_process().

Signed-off-by: Oleg Nesterov <oleg at redhat.com>
Cc: Sergey Dyasly <dserrg at gmail.com>
Cc: Sameer Nanda <snanda at chromium.org>
Cc: "Eric W. Biederman" <ebiederm at xmission.com>
Cc: Frederic Weisbecker <fweisbec at gmail.com>
Cc: Mandeep Singh Baines <msb at chromium.org>
Cc: "Ma, Xindong" <xindong.ma at intel.com>
Reviewed-by: Michal Hocko <mhocko at suse.cz>
Cc: "Tu, Xiaobing" <xiaobing.tu at intel.com>
Acked-by: David Rientjes <rientjes at google.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit 4d4048be8a93769350efa31d2482a038b7de73d0)
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/oom_kill.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 8e02a2e..af2d89e 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -102,14 +102,19 @@ struct task_struct *find_lock_task_mm(struct task_struct *p)
 {
 	struct task_struct *t;
 
+	rcu_read_lock();
+
 	for_each_thread(p, t) {
 		task_lock(t);
 		if (likely(t->mm))
-			return t;
+			goto found;
 		task_unlock(t);
 	}
+	t = NULL;
+found:
+	rcu_read_unlock();
 
-	return NULL;
+	return t;
 }
 
 /* return true if the task is not adequate as candidate victim task. */
@@ -478,10 +483,8 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	}
 	read_unlock(&tasklist_lock);
 
-	rcu_read_lock();
 	p = find_lock_task_mm(victim);
 	if (!p) {
-		rcu_read_unlock();
 		put_task_struct(victim);
 		return;
 	} else if (victim != p) {
@@ -507,6 +510,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 * That thread will now get access to memory reserves since it has a
 	 * pending fatal signal.
 	 */
+	rcu_read_lock();
 	for_each_process(p)
 		if (p->mm == mm && !same_thread_group(p, victim) &&
 		    !(p->flags & PF_KTHREAD)) {
-- 
2.7.4