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>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 453100a4159d3eed6b82307f7fed09f12eec1937..64b0e73402dfc412f721869735fe9c712d3d925a 100644 (file)
--- a/kernel/cgroup.c
+++ b/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.