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>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 92bcf1db16b24ea2c9b4d9f6c93043118c021615..a7b2460e922b779252ebcc225cecaf6060b0e964 100644 (file)
--- 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);
 }