cgroup: Migrate tasks to the root css when a controller is rebound

cgroup_apply_control_disable() defers kill_css_finish() while a css is
still populated, relying on css_update_populated() to fire the deferred
kill once the populated count reaches zero.

This deadlocks when a controller is rebound out of a hierarchy. Mounting
an implicit_on_dfl controller such as perf_event as a v1 hierarchy steals
it off the default hierarchy, and rebind_subsystems() kills its
per-cgroup csses while they are still populated. The migration run in the
same step keeps the old css for a controller no longer in the hierarchy's
mask, so no task is migrated off the dying csses. Their populated count
never reaches zero, the deferred kill_css_finish() never fires, and the
next cgroup_lock_and_drain_offline() hangs forever under cgroup_mutex.

That migration is already a no-op pass over the rebound subtree. Add
cgroup_rebind_ss_mask so find_existing_css_set() resolves the leaving
controllers to the root css. Their tasks are migrated there, the
per-cgroup csses depopulate, and cgroup_apply_control_disable() kills
them synchronously. The deferral stays correct for the rmdir and
controller-disable paths it was meant for.

Fixes: 1dffd95 ("cgroup: Defer kill_css_finish() in cgroup_apply_control_disable()")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/41cd159c-54e5-45e0-81df-eaf36a6c028e@sirena.org.uk/
Reported-by: Bert Karwatzki <spasswolf@web.de>
Closes: https://lore.kernel.org/all/4e986b4ed7e16547805d54b6e67d09120bc4d2f2.camel@web.de/
Tested-by: Mark Brown <broonie@kernel.org>
Tested-by: Bert Karwatzki <spasswolf@web.de>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

kernel/cgroup/cgroup.c

@@ -197,6 +197,14 @@ static u32 cgrp_dfl_implicit_ss_mask;
 /* some controllers can be threaded on the default hierarchy */
 static u32 cgrp_dfl_threaded_ss_mask;
 
+/*
+ * Set across rebind_subsystems() to the controllers leaving a hierarchy.
+ * Guarded by cgroup_mutex. Makes find_existing_css_set() resolve them to the
+ * root css so the affected tasks are migrated there before
+ * cgroup_apply_control_disable() kills the per-cgroup csses.
+ */
+static u32 cgroup_rebind_ss_mask;
+
 /* The list of hierarchy roots */
 LIST_HEAD(cgroup_roots);
 static int cgroup_root_count;
@@ -1083,7 +1091,15 @@ static struct css_set *find_existing_css_set(struct css_set *old_cset,
 	 * won't change, so no need for locking.
 	 */
 	for_each_subsys(ss, i) {
-		if (root->subsys_mask & (1UL << i)) {
+		if (unlikely(cgroup_rebind_ss_mask & (1UL << i))) {
+			/*
+			 * @ss is leaving this hierarchy and its per-cgroup
+			 * csses are about to be killed. Resolve to the
+			 * surviving root css so the tasks are migrated there.
+			 */
+			template[i] = cgroup_css(&root->cgrp, ss);
+			WARN_ON_ONCE(!template[i]);
+		} else if (root->subsys_mask & (1UL << i)) {
 			/*
 			 * @ss is in this hierarchy, so we want the
 			 * effective css from @cgrp.
@@ -1853,11 +1869,17 @@ int rebind_subsystems(struct cgroup_root *dst_root, u32 ss_mask)
 		struct cgroup *scgrp = &cgrp_dfl_root.cgrp;
 
 		/*
-		 * Controllers from default hierarchy that need to be rebound
-		 * are all disabled together in one go.
+		 * Controllers leaving the default hierarchy are disabled
+		 * together. cgroup_rebind_ss_mask makes cgroup_apply_control()
+		 * migrate their tasks to the root css, so the per-cgroup csses
+		 * are unpopulated when cgroup_finalize_control() kills them.
+		 * Clear it before cgroup_finalize_control(), which does no
+		 * css_set lookup.
 		 */
 		cgrp_dfl_root.subsys_mask &= ~dfl_disable_ss_mask;
+		cgroup_rebind_ss_mask = dfl_disable_ss_mask;
 		WARN_ON(cgroup_apply_control(scgrp));
+		cgroup_rebind_ss_mask = 0;
 		cgroup_finalize_control(scgrp, 0);
 	}
 
@@ -1871,9 +1893,14 @@ int rebind_subsystems(struct cgroup_root *dst_root, u32 ss_mask)
 		WARN_ON(!css || cgroup_css(dcgrp, ss));
 
 		if (src_root != &cgrp_dfl_root) {
-			/* disable from the source */
+			/*
+			 * Disable from the source, migrating its tasks to the
+			 * root css first (see cgroup_rebind_ss_mask).
+			 */
 			src_root->subsys_mask &= ~(1 << ssid);
+			cgroup_rebind_ss_mask = 1 << ssid;
 			WARN_ON(cgroup_apply_control(scgrp));
+			cgroup_rebind_ss_mask = 0;
 			cgroup_finalize_control(scgrp, 0);
 		}