[PATCH 2/2] mm/linux-next: Fix rcu locking in vm_is_stack

[Siddhesh Poyarekar <siddhesh.poyarekar@xxxxxxxxx>]

Take rcu read lock before we do anything at all with the threadgroup
list. Also use list_first_entry_rcu to safely get the reference to the
first task in the list.

Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@xxxxxxxxx>
---
 mm/memory.c |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0ca7fe6..1d5830c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3932,18 +3932,20 @@ pid_t vm_is_stack(struct task_struct *task,
 		return task->pid;
 
 	if (in_group) {
-		struct task_struct *t = task;
+		struct task_struct *t;
 		rcu_read_lock();
-		while_each_thread(task, t) {
+		t = list_first_entry_rcu(&task->thread_group,
+					 struct task_struct, thread_group);
+		do {
 			if (vm_is_stack_for_task(t, vma)) {
 				ret = t->pid;
 				goto done;
 			}
-		}
+		} while_each_thread(task, t);
+done:
+		rcu_read_unlock();
 	}
 
-done:
-	rcu_read_unlock();
 	return ret;
 }
 
-- 
1.7.7.4

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