blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()

We've triggered a WARNING in blk_throtl_bio() when throttling writeback
io, which complains blkg->refcnt is already 0 when calling blkg_get(),
and then kernel crashes with invalid page request.
After investigating this issue, we've found it is caused by a race
between blkcg_bio_issue_check() and cgroup_rmdir(), which is described
below:

writeback kworker               cgroup_rmdir
                                  cgroup_destroy_locked
                                    kill_css
                                      css_killed_ref_fn
                                        css_killed_work_fn
                                          offline_css
                                            blkcg_css_offline
  blkcg_bio_issue_check
    rcu_read_lock
    blkg_lookup
                                              spin_trylock(q->queue_lock)
                                              blkg_destroy
                                              spin_unlock(q->queue_lock)
    blk_throtl_bio
    spin_lock_irq(q->queue_lock)
    ...
    spin_unlock_irq(q->queue_lock)
  rcu_read_unlock

Since rcu can only prevent blkg from releasing when it is being used,
the blkg->refcnt can be decreased to 0 during blkg_destroy() and schedule
blkg release.
Then trying to blkg_get() in blk_throtl_bio() will complains the WARNING.
And then the corresponding blkg_put() will schedule blkg release again,
which result in double free.
This race is introduced by commit ae11889636 ("blkcg: consolidate blkg
creation in blkcg_bio_issue_check()"). Before this commit, it will
lookup first and then try to lookup/create again with queue_lock. Since
revive this logic is a bit drastic, so fix it by only offlining pd during
blkcg_css_offline(), and move the rest destruction (especially
blkg_put()) into blkcg_css_free(), which should be the right way as
discussed.

Fixes: ae11889636 ("blkcg: consolidate blkg creation in blkcg_bio_issue_check()")
Reported-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -307,11 +307,28 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 	}
 }
 
+static void blkg_pd_offline(struct blkcg_gq *blkg)
+{
+	int i;
+
+	lockdep_assert_held(blkg->q->queue_lock);
+	lockdep_assert_held(&blkg->blkcg->lock);
+
+	for (i = 0; i < BLKCG_MAX_POLS; i++) {
+		struct blkcg_policy *pol = blkcg_policy[i];
+
+		if (blkg->pd[i] && !blkg->pd[i]->offline &&
+		    pol->pd_offline_fn) {
+			pol->pd_offline_fn(blkg->pd[i]);
+			blkg->pd[i]->offline = true;
+		}
+	}
+}
+
 static void blkg_destroy(struct blkcg_gq *blkg)
 {
 	struct blkcg *blkcg = blkg->blkcg;
 	struct blkcg_gq *parent = blkg->parent;
-	int i;
 
 	lockdep_assert_held(blkg->q->queue_lock);
 	lockdep_assert_held(&blkcg->lock);
@@ -320,13 +337,6 @@ static void blkg_destroy(struct blkcg_gq *blkg)
 	WARN_ON_ONCE(list_empty(&blkg->q_node));
 	WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));
 
-	for (i = 0; i < BLKCG_MAX_POLS; i++) {
-		struct blkcg_policy *pol = blkcg_policy[i];
-
-		if (blkg->pd[i] && pol->pd_offline_fn)
-			pol->pd_offline_fn(blkg->pd[i]);
-	}
-
 	if (parent) {
 		blkg_rwstat_add_aux(&parent->stat_bytes, &blkg->stat_bytes);
 		blkg_rwstat_add_aux(&parent->stat_ios, &blkg->stat_ios);
@@ -369,6 +379,7 @@ static void blkg_destroy_all(struct request_queue *q)
 		struct blkcg *blkcg = blkg->blkcg;
 
 		spin_lock(&blkcg->lock);
+		blkg_pd_offline(blkg);
 		blkg_destroy(blkg);
 		spin_unlock(&blkcg->lock);
 	}
@@ -995,25 +1006,25 @@ static struct cftype blkcg_legacy_files[] = {
  * @css: css of interest
  *
  * This function is called when @css is about to go away and responsible
- * for shooting down all blkgs associated with @css.  blkgs should be
+ * for offlining all blkgs pd and killing all wbs associated with @css.
- * removed while holding both q and blkcg locks.  As blkcg lock is nested
+ * blkgs pd offline should be done while holding both q and blkcg locks.
- * inside q lock, this function performs reverse double lock dancing.
+ * As blkcg lock is nested inside q lock, this function performs reverse
+ * double lock dancing.
  *
  * This is the blkcg counterpart of ioc_release_fn().
  */
 static void blkcg_css_offline(struct cgroup_subsys_state *css)
 {
 	struct blkcg *blkcg = css_to_blkcg(css);
+	struct blkcg_gq *blkg;
 
 	spin_lock_irq(&blkcg->lock);
 
-	while (!hlist_empty(&blkcg->blkg_list)) {
+	hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
-		struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
-						struct blkcg_gq, blkcg_node);
 		struct request_queue *q = blkg->q;
 
 		if (spin_trylock(q->queue_lock)) {
-			blkg_destroy(blkg);
+			blkg_pd_offline(blkg);
 			spin_unlock(q->queue_lock);
 		} else {
 			spin_unlock_irq(&blkcg->lock);
@@ -1027,11 +1038,43 @@ static void blkcg_css_offline(struct cgroup_subsys_state *css)
 	wb_blkcg_offline(blkcg);
 }
 
+/**
+ * blkcg_destroy_all_blkgs - destroy all blkgs associated with a blkcg
+ * @blkcg: blkcg of interest
+ *
+ * This function is called when blkcg css is about to free and responsible for
+ * destroying all blkgs associated with @blkcg.
+ * blkgs should be removed while holding both q and blkcg locks. As blkcg lock
+ * is nested inside q lock, this function performs reverse double lock dancing.
+ */
+static void blkcg_destroy_all_blkgs(struct blkcg *blkcg)
+{
+	spin_lock_irq(&blkcg->lock);
+	while (!hlist_empty(&blkcg->blkg_list)) {
+		struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
+						    struct blkcg_gq,
+						    blkcg_node);
+		struct request_queue *q = blkg->q;
+
+		if (spin_trylock(q->queue_lock)) {
+			blkg_destroy(blkg);
+			spin_unlock(q->queue_lock);
+		} else {
+			spin_unlock_irq(&blkcg->lock);
+			cpu_relax();
+			spin_lock_irq(&blkcg->lock);
+		}
+	}
+	spin_unlock_irq(&blkcg->lock);
+}
+
 static void blkcg_css_free(struct cgroup_subsys_state *css)
 {
 	struct blkcg *blkcg = css_to_blkcg(css);
 	int i;
 
+	blkcg_destroy_all_blkgs(blkcg);
+
 	mutex_lock(&blkcg_pol_mutex);
 
 	list_del(&blkcg->all_blkcgs_node);
@@ -1371,8 +1414,11 @@ void blkcg_deactivate_policy(struct request_queue *q,
 		spin_lock(&blkg->blkcg->lock);
 
 		if (blkg->pd[pol->plid]) {
-			if (pol->pd_offline_fn)
+			if (!blkg->pd[pol->plid]->offline &&
+			    pol->pd_offline_fn) {
 				pol->pd_offline_fn(blkg->pd[pol->plid]);
+				blkg->pd[pol->plid]->offline = true;
+			}
 			pol->pd_free_fn(blkg->pd[pol->plid]);
 			blkg->pd[pol->plid] = NULL;
 		}

include/linux/blk-cgroup.h

@@ -88,6 +88,7 @@ struct blkg_policy_data {
 	/* the blkg and policy id this per-policy data belongs to */
 	struct blkcg_gq			*blkg;
 	int				plid;
+	bool				offline;
 };
 
 /*