[PATCH 2/2] cgroup: Use separate work structs on css release path

["Michal Koutný" <mkoutny-IBi9RG/b67k@public.gmane.org>]

The cgroup_subsys_state of cgroup subsystems (not cgroup->self) use both
kill and release callbacks on their release path (see comment for
css_free_rwork_fn()).

When the last reference is also the base reference, we run into issues
when active work_struct (1) is re-initialized from css_release (2).

// ref=1: only base reference
kill_css()
  css_get() // fuse, ref+=1 == 2
  percpu_ref_kill_and_confirm
    // ref -= 1 == 1: kill base references
  [via rcu]
  css_killed_ref_fn == refcnt.confirm_switch
    queue_work(css->destroy_work)               (1)
    [via css->destroy_work]
    css_killed_work_fn == wq.func
      offline_css() // needs fuse
      css_put // ref -= 1 == 0: de-fuse, was last
        ...
        percpu_ref_put_many
           css_release
             queue_work(css->destroy_work)      (2)
             [via css->destroy_work]
             css_release_work_fn == wq.func

Despite we take a fuse reference in css_killed_work_fn() it serves
for pinning the css until only after offline_css().

We could check inside css_release whether destroy_work is active
(WORK_STRUCT_PENDING_BIT) and daisy-chain css_release_work_fn from
css_release(). In order to avoid clashes with various stages of the work
item processing, we just spend some space in css (my config's css grows
to 232B + 32B) and create a separate work entry for each user.

Reported-by: syzbot+e42ae441c3b10acf9e9d-Pl5Pbv+GP7P466ipTTIvnc23WoclnBCfAL8bYrjMMd8@public.gmane.org
Reported-by: Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
Link: https://lore.kernel.org/r/20220412192459.227740-1-tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org/
Signed-off-by: Tadeusz Struk <tadeusz.struk-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
Signed-off-by: Michal Koutn√Ω <mkoutny-IBi9RG/b67k@public.gmane.org>
---
 include/linux/cgroup-defs.h |  5 +++--
 kernel/cgroup/cgroup.c      | 14 +++++++-------
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 1bfcfb1af352..16b99aa04305 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -178,8 +178,9 @@ struct cgroup_subsys_state {
 	 */
 	atomic_t online_cnt;
 
-	/* percpu_ref killing and RCU release */
-	struct work_struct destroy_work;
+	/* percpu_ref killing, css release, and RCU release work structs */
+	struct work_struct killed_ref_work;
+	struct work_struct release_work;
 	struct rcu_work destroy_rwork;
 
 	/*
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index a5b0d5d54fbc..33b3a44391d7 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5102,7 +5102,7 @@ static struct cftype cgroup_base_files[] = {
  *    css_free_work_fn().
  *
  * It is actually hairier because both step 2 and 4 require process context
- * and thus involve punting to css->destroy_work adding two additional
+ * and thus involve punting to css->release_work adding two additional
  * steps to the already complex sequence.
  */
 static void css_free_rwork_fn(struct work_struct *work)
@@ -5157,7 +5157,7 @@ static void css_free_rwork_fn(struct work_struct *work)
 static void css_release_work_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =
-		container_of(work, struct cgroup_subsys_state, destroy_work);
+		container_of(work, struct cgroup_subsys_state, release_work);
 	struct cgroup_subsys *ss = css->ss;
 	struct cgroup *cgrp = css->cgroup;
 
@@ -5213,8 +5213,8 @@ static void css_release(struct percpu_ref *ref)
 	struct cgroup_subsys_state *css =
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
-	INIT_WORK(&css->destroy_work, css_release_work_fn);
-	queue_work(cgroup_destroy_wq, &css->destroy_work);
+	INIT_WORK(&css->release_work, css_release_work_fn);
+	queue_work(cgroup_destroy_wq, &css->release_work);
 }
 
 static void init_and_link_css(struct cgroup_subsys_state *css,
@@ -5549,7 +5549,7 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
 static void css_killed_work_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =
-		container_of(work, struct cgroup_subsys_state, destroy_work);
+		container_of(work, struct cgroup_subsys_state, killed_ref_work);
 
 	mutex_lock(&cgroup_mutex);
 
@@ -5570,8 +5570,8 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
 		container_of(ref, struct cgroup_subsys_state, refcnt);
 
 	if (atomic_dec_and_test(&css->online_cnt)) {
-		INIT_WORK(&css->destroy_work, css_killed_work_fn);
-		queue_work(cgroup_destroy_wq, &css->destroy_work);
+		INIT_WORK(&css->killed_ref_work, css_killed_work_fn);
+		queue_work(cgroup_destroy_wq, &css->killed_ref_work);
 	}
 }
 
-- 
2.35.3