oom: fix possible oom_dump_tasks NULL pointer

When /proc/sys/vm/oom_dump_tasks is enabled, it is possible to get a NULL
pointer for tasks that have detached mm's since task_lock() is not held
during the tasklist scan.  Add the task_lock().

Acked-by: Nick Piggin <npiggin@suse.de>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-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, 15 insertions, 9 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 92bcf1db16b2..a7b2460e922b 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -284,22 +284,28 @@ static void dump_tasks(const struct mem_cgroup *mem)
         printk(KERN_INFO "[ pid ]   uid  tgid total_vm      rss cpu oom_adj "
                "name\n");
         do_each_thread(g, p) {
-                /*
-                 * total_vm and rss sizes do not exist for tasks with a
-                 * detached mm so there's no need to report them.
-                 */
-                if (!p->mm)
-                        continue;
+                struct mm_struct *mm;
+
                 if (mem && !task_in_mem_cgroup(p, mem))
                         continue;
                 if (!thread_group_leader(p))
                         continue;
 
                 task_lock(p);
+                mm = p->mm;
+                if (!mm) {
+                        /*
+                         * total_vm and rss sizes do not exist for tasks with no
+                         * mm so there's no need to report them; they can't be
+                         * oom killed anyway.
+                         */
+                        task_unlock(p);
+                        continue;
+                }
                 printk(KERN_INFO "[%5d] %5d %5d %8lu %8lu %3d     %3d %s\n",
-                       p->pid, __task_cred(p)->uid, p->tgid,
-                       p->mm->total_vm, get_mm_rss(p->mm), (int)task_cpu(p),
-                       p->oomkilladj, p->comm);
+                       p->pid, __task_cred(p)->uid, p->tgid, mm->total_vm,
+                       get_mm_rss(mm), (int)task_cpu(p), p->oomkilladj,
+                       p->comm);
                 task_unlock(p);
         } while_each_thread(g, p);
 }