cgroups: don't attach task to subsystem if migration failed

If a task has exited to the point it has called cgroup_exit() already,
then we can't migrate it to another cgroup anymore.

This can happen when we are attaching a task to a new cgroup between the
call to ->can_attach_task() on subsystems and the migration that is
eventually tried in cgroup_task_migrate().

In this case cgroup_task_migrate() returns -ESRCH and we don't want to
attach the task to the subsystems because the attachment to the new cgroup
itself failed.

Fix this by only calling ->attach_task() on the subsystems if the cgroup
migration succeeded.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
Acked-by: Paul Menage <paul@paulmenage.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 64b0e73402dfc412f721869735fe9c712d3d925a..8386b21224ef3e03ecc0ed850d949d6345888448 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2135,14 +2135,17 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                oldcgrp = task_cgroup_from_root(tsk, root);
                if (cgrp == oldcgrp)
                        continue;
-               /* attach each task to each subsystem */
-               for_each_subsys(root, ss) {
-                       if (ss->attach_task)
-                               ss->attach_task(cgrp, tsk);
-               }
                /* if the thread is PF_EXITING, it can just get skipped. */
                retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, true);
-               BUG_ON(retval != 0 && retval != -ESRCH);
+               if (retval == 0) {
+                       /* attach each task to each subsystem */
+                       for_each_subsys(root, ss) {
+                               if (ss->attach_task)
+                                       ss->attach_task(cgrp, tsk);
+                       }
+               } else {
+                       BUG_ON(retval != -ESRCH);
+               }
        }
        /* nothing is sensitive to fork() after this point. */