oom_kill: add rcu_read_lock() into find_lock_task_mm()

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@redhat.com>
Cc: Sergey Dyasly <dserrg@gmail.com>
Cc: Sameer Nanda <snanda@chromium.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mandeep Singh Baines <msb@chromium.org>
Cc: "Ma, Xindong" <xindong.ma@intel.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: "Tu, Xiaobing" <xiaobing.tu@intel.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 0d8ad1ebd1d1..054ff47c4478 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. */
@@ -461,10 +466,8 @@ 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) {
@@ -490,6 +493,7 @@ 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)) {