mm/mempolicy.c: add rcu read lock to protect pid structure

find_task_by_vpid() should be protected by rcu_read_lock(), to prevent
free_pid() reclaiming pid.
Signed-off-by: Zeng Zhaoming <zengzm.kernel@gmail.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1307,15 +1307,18 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
 		goto out;
 
 	/* Find the mm_struct */
+	rcu_read_lock();
 	read_lock(&tasklist_lock);
 	task = pid ? find_task_by_vpid(pid) : current;
 	if (!task) {
 		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 		err = -ESRCH;
 		goto out;
 	}
 	mm = get_task_mm(task);
 	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 
 	err = -EINVAL;
 	if (!mm)