[patch] oom: fix possible oom_dump_tasks NULL pointer

[David Rientjes]

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@xxxxxxx>
Acked-by: Mel Gorman <mel@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/oom_kill.c |   24 +++++++++++++++---------
 1 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
--- 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);
 }
--
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