cgroup: move migration destination verification out of cgroup_migrate_prepare_dst()
cgroup_migrate_prepare_dst() verifies whether the destination cgroup
is allowable; however, the test doesn't really belong there. It's too
deep and common in the stack and as a result the test itself is gated
by another test.
Separate the test out into cgroup_may_migrate_to() and update
cgroup_attach_task() and cgroup_transfer_tasks() to perform the test
directly. This doesn't cause any behavior differences.
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 50879aa..8a02076 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2444,6 +2444,20 @@
}
/**
+ * cgroup_may_migrate_to - verify whether a cgroup can be migration destination
+ * @dst_cgrp: destination cgroup to test
+ *
+ * On the default hierarchy, except for the root, subtree_control must be
+ * zero for migration destination cgroups with tasks so that child cgroups
+ * don't compete against tasks.
+ */
+static bool cgroup_may_migrate_to(struct cgroup *dst_cgrp)
+{
+ return !cgroup_on_dfl(dst_cgrp) || !cgroup_parent(dst_cgrp) ||
+ !dst_cgrp->subtree_control;
+}
+
+/**
* cgroup_migrate_finish - cleanup after attach
* @preloaded_csets: list of preloaded css_sets
*
@@ -2529,14 +2543,6 @@
lockdep_assert_held(&cgroup_mutex);
- /*
- * Except for the root, subtree_control must be zero for a cgroup
- * with tasks so that child cgroups don't compete against tasks.
- */
- if (dst_cgrp && cgroup_on_dfl(dst_cgrp) && cgroup_parent(dst_cgrp) &&
- dst_cgrp->subtree_control)
- return -EBUSY;
-
/* look up the dst cset for each src cset and link it to src */
list_for_each_entry_safe(src_cset, tmp_cset, preloaded_csets, mg_preload_node) {
struct css_set *dst_cset;
@@ -2634,6 +2640,9 @@
struct task_struct *task;
int ret;
+ if (!cgroup_may_migrate_to(dst_cgrp))
+ return -EBUSY;
+
/* look up all src csets */
spin_lock_bh(&css_set_lock);
rcu_read_lock();
@@ -4136,6 +4145,9 @@
struct task_struct *task;
int ret;
+ if (!cgroup_may_migrate_to(to))
+ return -EBUSY;
+
mutex_lock(&cgroup_mutex);
/* all tasks in @from are being moved, all csets are source */