cgroup: Fix dom_cgrp propagation when enabling threaded mode

A cgroup which is already a threaded domain may be converted into a
threaded cgroup if the prerequisite conditions are met.  When this
happens, all threaded descendant should also have their ->dom_cgrp
updated to the new threaded domain cgroup.  Unfortunately, this
propagation was missing leading to the following failure.

  # cd /sys/fs/cgroup/unified
  # cat cgroup.subtree_control    # show that no controllers are enabled

  # mkdir -p mycgrp/a/b/c
  # echo threaded > mycgrp/a/b/cgroup.type

  At this point, the hierarchy looks as follows:

      mycgrp [d]
	  a [dt]
	      b [t]
		  c [inv]

  Now let's make node "a" threaded (and thus "mycgrp" s made "domain threaded"):

  # echo threaded > mycgrp/a/cgroup.type

  By this point, we now have a hierarchy that looks as follows:

      mycgrp [dt]
	  a [t]
	      b [t]
		  c [inv]

  But, when we try to convert the node "c" from "domain invalid" to
  "threaded", we get ENOTSUP on the write():

  # echo threaded > mycgrp/a/b/c/cgroup.type
  sh: echo: write error: Operation not supported

This patch fixes the problem by

* Moving the opencoded ->dom_cgrp save and restoration in
  cgroup_enable_threaded() into cgroup_{save|restore}_control() so
  that mulitple cgroups can be handled.

* Updating all threaded descendants' ->dom_cgrp to point to the new
  dom_cgrp when enabling threaded mode.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Reported-by: Amin Jamali <ajamali@pivotal.io>
Reported-by: Joao De Almeida Pereira <jpereira@pivotal.io>
Link: https://lore.kernel.org/r/CAKgNAkhHYCMn74TCNiMJ=ccLd7DcmXSbvw3CbZ1YREeG7iJM5g@mail.gmail.com
Fixes: 454000ad ("cgroup: introduce cgroup->dom_cgrp and threaded css_set handling")
Cc: stable@vger.kernel.org # v4.14+

include/linux/cgroup-defs.h

@@ -412,6 +412,7 @@ struct cgroup {
 	 * specific task are charged to the dom_cgrp.
 	 */
 	struct cgroup *dom_cgrp;
+	struct cgroup *old_dom_cgrp;		/* used while enabling threaded */
 
 	/* per-cpu recursive resource statistics */
 	struct cgroup_rstat_cpu __percpu *rstat_cpu;

kernel/cgroup/cgroup.c

@@ -2836,11 +2836,12 @@ void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
 }
 
 /**
- * cgroup_save_control - save control masks of a subtree
+ * cgroup_save_control - save control masks and dom_cgrp of a subtree
  * @cgrp: root of the target subtree
  *
- * Save ->subtree_control and ->subtree_ss_mask to the respective old_
- * prefixed fields for @cgrp's subtree including @cgrp itself.
+ * Save ->subtree_control, ->subtree_ss_mask and ->dom_cgrp to the
+ * respective old_ prefixed fields for @cgrp's subtree including @cgrp
+ * itself.
  */
 static void cgroup_save_control(struct cgroup *cgrp)
 {
@@ -2850,6 +2851,7 @@ static void cgroup_save_control(struct cgroup *cgrp)
 	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
 		dsct->old_subtree_control = dsct->subtree_control;
 		dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
+		dsct->old_dom_cgrp = dsct->dom_cgrp;
 	}
 }
 
@@ -2875,11 +2877,12 @@ static void cgroup_propagate_control(struct cgroup *cgrp)
 }
 
 /**
- * cgroup_restore_control - restore control masks of a subtree
+ * cgroup_restore_control - restore control masks and dom_cgrp of a subtree
  * @cgrp: root of the target subtree
  *
- * Restore ->subtree_control and ->subtree_ss_mask from the respective old_
- * prefixed fields for @cgrp's subtree including @cgrp itself.
+ * Restore ->subtree_control, ->subtree_ss_mask and ->dom_cgrp from the
+ * respective old_ prefixed fields for @cgrp's subtree including @cgrp
+ * itself.
  */
 static void cgroup_restore_control(struct cgroup *cgrp)
 {
@@ -2889,6 +2892,7 @@ static void cgroup_restore_control(struct cgroup *cgrp)
 	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
 		dsct->subtree_control = dsct->old_subtree_control;
 		dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
+		dsct->dom_cgrp = dsct->old_dom_cgrp;
 	}
 }
 
@@ -3196,6 +3200,8 @@ static int cgroup_enable_threaded(struct cgroup *cgrp)
 {
 	struct cgroup *parent = cgroup_parent(cgrp);
 	struct cgroup *dom_cgrp = parent->dom_cgrp;
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
 	int ret;
 
 	lockdep_assert_held(&cgroup_mutex);
@@ -3225,12 +3231,13 @@ static int cgroup_enable_threaded(struct cgroup *cgrp)
 	 */
 	cgroup_save_control(cgrp);
 
-	cgrp->dom_cgrp = dom_cgrp;
+	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)
+		if (dsct == cgrp || cgroup_is_threaded(dsct))
+			dsct->dom_cgrp = dom_cgrp;
+
 	ret = cgroup_apply_control(cgrp);
 	if (!ret)
 		parent->nr_threaded_children++;
-	else
-		cgrp->dom_cgrp = cgrp;
 
 	cgroup_finalize_control(cgrp, ret);
 	return ret;