cgroups: more safe tasklist locking in cgroup_attach_proc

Fix unstable tasklist locking in cgroup_attach_proc.

According to this thread - https://lkml.org/lkml/2011/7/27/243 - RCU is
not sufficient to guarantee the tasklist is stable w.r.t.  de_thread and
exit.  Taking tasklist_lock for reading, instead of rcu_read_lock, ensures
proper exclusion.
Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
Acked-by: Paul Menage <paul@paulmenage.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup.c

@@ -2027,7 +2027,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		goto out_free_group_list;
 
 	/* prevent changes to the threadgroup list while we take a snapshot. */
-	rcu_read_lock();
+	read_lock(&tasklist_lock);
 	if (!thread_group_leader(leader)) {
 		/*
 		 * a race with de_thread from another thread's exec() may strip
@@ -2036,7 +2036,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		 * throw this task away and try again (from cgroup_procs_write);
 		 * this is "double-double-toil-and-trouble-check locking".
 		 */
-		rcu_read_unlock();
+		read_unlock(&tasklist_lock);
 		retval = -EAGAIN;
 		goto out_free_group_list;
 	}
@@ -2057,7 +2057,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	} while_each_thread(leader, tsk);
 	/* remember the number of threads in the array for later. */
 	group_size = i;
-	rcu_read_unlock();
+	read_unlock(&tasklist_lock);
 
 	/*
 	 * step 1: check that we can legitimately attach to the cgroup.