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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 6 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 64b0e73402df..8386b21224ef 100644
--- 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. */