blkcg: revert blkcg cleanups series

This reverts a series committed earlier due to null pointer exception
bug report in [1]. It seems there are edge case interactions that I did
not consider and will need some time to understand what causes the
adverse interactions.

The original series can be found in [2] with a follow up series in [3].

[1] https://www.spinics.net/lists/cgroups/msg20719.html
[2] https://lore.kernel.org/lkml/20180911184137.35897-1-dennisszhou@gmail.com/
[3] https://lore.kernel.org/lkml/20181020185612.51587-1-dennis@kernel.org/

This reverts the following commits:
d459d853c2ed, b2c3fa546705, 101246ec02b5, b3b9f24f5fcc, e2b0989954ae,
f0fcb3ec89f3, c839e7a03f92, bdc2491708c4, 74b7c02a9bc1, 5bf9a1f3b4ef,
a7b39b4e961c, 07b05bcc3213, 49f4c2dc2b50, 27e6fa996c53

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 38 insertions, 85 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 992da5592c6e..c630e02836a8 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -84,37 +84,6 @@ static void blkg_free(struct blkcg_gq *blkg)
 	kfree(blkg);
 }
 
-static void __blkg_release(struct rcu_head *rcu)
-{
-	struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);
-
-	percpu_ref_exit(&blkg->refcnt);
-
-	/* release the blkcg and parent blkg refs this blkg has been holding */
-	css_put(&blkg->blkcg->css);
-	if (blkg->parent)
-		blkg_put(blkg->parent);
-
-	wb_congested_put(blkg->wb_congested);
-
-	blkg_free(blkg);
-}
-
-/*
- * A group is RCU protected, but having an rcu lock does not mean that one
- * can access all the fields of blkg and assume these are valid.  For
- * example, don't try to follow throtl_data and request queue links.
- *
- * Having a reference to blkg under an rcu allows accesses to only values
- * local to groups like group stats and group rate limits.
- */
-static void blkg_release(struct percpu_ref *ref)
-{
-	struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);
-
-	call_rcu(&blkg->rcu_head, __blkg_release);
-}
-
 /**
  * blkg_alloc - allocate a blkg
  * @blkcg: block cgroup the new blkg is associated with
@@ -141,6 +110,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
 	blkg->q = q;
 	INIT_LIST_HEAD(&blkg->q_node);
 	blkg->blkcg = blkcg;
+	atomic_set(&blkg->refcnt, 1);
 
 	/* root blkg uses @q->root_rl, init rl only for !root blkgs */
 	if (blkcg != &blkcg_root) {
@@ -247,11 +217,6 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 		blkg_get(blkg->parent);
 	}
 
-	ret = percpu_ref_init(&blkg->refcnt, blkg_release, 0,
-			      GFP_NOWAIT | __GFP_NOWARN);
-	if (ret)
-		goto err_cancel_ref;
-
 	/* invoke per-policy init */
 	for (i = 0; i < BLKCG_MAX_POLS; i++) {
 		struct blkcg_policy *pol = blkcg_policy[i];
@@ -284,8 +249,6 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
 	blkg_put(blkg);
 	return ERR_PTR(ret);
 
-err_cancel_ref:
-	percpu_ref_exit(&blkg->refcnt);
 err_put_congested:
 	wb_congested_put(wb_congested);
 err_put_css:
@@ -296,7 +259,7 @@ err_free_blkg:
 }
 
 /**
- * __blkg_lookup_create - lookup blkg, try to create one if not there
+ * blkg_lookup_create - lookup blkg, try to create one if not there
  * @blkcg: blkcg of interest
  * @q: request_queue of interest
  *
@@ -305,11 +268,12 @@ err_free_blkg:
  * that all non-root blkg's have access to the parent blkg.  This function
  * should be called under RCU read lock and @q->queue_lock.
  *
- * Returns the blkg or the closest blkg if blkg_create fails as it walks
- * down from root.
+ * Returns pointer to the looked up or created blkg on success, ERR_PTR()
+ * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
+ * dead and bypassing, returns ERR_PTR(-EBUSY).
  */
-struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
-				      struct request_queue *q)
+struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
+				    struct request_queue *q)
 {
 	struct blkcg_gq *blkg;
 
@@ -321,7 +285,7 @@ struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
 	 * we shouldn't allow anything to go through for a bypassing queue.
 	 */
 	if (unlikely(blk_queue_bypass(q)))
-		return q->root_blkg;
+		return ERR_PTR(blk_queue_dying(q) ? -ENODEV : -EBUSY);
 
 	blkg = __blkg_lookup(blkcg, q, true);
 	if (blkg)
@@ -329,58 +293,23 @@ struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
 
 	/*
 	 * Create blkgs walking down from blkcg_root to @blkcg, so that all
-	 * non-root blkgs have access to their parents.  Returns the closest
-	 * blkg to the intended blkg should blkg_create() fail.
+	 * non-root blkgs have access to their parents.
 	 */
 	while (true) {
 		struct blkcg *pos = blkcg;
 		struct blkcg *parent = blkcg_parent(blkcg);
-		struct blkcg_gq *ret_blkg = q->root_blkg;
-
-		while (parent) {
-			blkg = __blkg_lookup(parent, q, false);
-			if (blkg) {
-				/* remember closest blkg */
-				ret_blkg = blkg;
-				break;
-			}
+
+		while (parent && !__blkg_lookup(parent, q, false)) {
 			pos = parent;
 			parent = blkcg_parent(parent);
 		}
 
 		blkg = blkg_create(pos, q, NULL);
-		if (IS_ERR(blkg))
-			return ret_blkg;
-		if (pos == blkcg)
+		if (pos == blkcg || IS_ERR(blkg))
 			return blkg;
 	}
 }
 
-/**
- * blkg_lookup_create - find or create a blkg
- * @blkcg: target block cgroup
- * @q: target request_queue
- *
- * This looks up or creates the blkg representing the unique pair
- * of the blkcg and the request_queue.
- */
-struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
-				    struct request_queue *q)
-{
-	struct blkcg_gq *blkg = blkg_lookup(blkcg, q);
-	unsigned long flags;
-
-	if (unlikely(!blkg)) {
-		spin_lock_irqsave(q->queue_lock, flags);
-
-		blkg = __blkg_lookup_create(blkcg, q);
-
-		spin_unlock_irqrestore(q->queue_lock, flags);
-	}
-
-	return blkg;
-}
-
 static void blkg_destroy(struct blkcg_gq *blkg)
 {
 	struct blkcg *blkcg = blkg->blkcg;
@@ -424,7 +353,7 @@ static void blkg_destroy(struct blkcg_gq *blkg)
 	 * Put the reference taken at the time of creation so that when all
 	 * queues are gone, group can be destroyed.
 	 */
-	percpu_ref_kill(&blkg->refcnt);
+	blkg_put(blkg);
 }
 
 /**
@@ -452,6 +381,29 @@ static void blkg_destroy_all(struct request_queue *q)
 }
 
 /*
+ * A group is RCU protected, but having an rcu lock does not mean that one
+ * can access all the fields of blkg and assume these are valid.  For
+ * example, don't try to follow throtl_data and request queue links.
+ *
+ * Having a reference to blkg under an rcu allows accesses to only values
+ * local to groups like group stats and group rate limits.
+ */
+void __blkg_release_rcu(struct rcu_head *rcu_head)
+{
+	struct blkcg_gq *blkg = container_of(rcu_head, struct blkcg_gq, rcu_head);
+
+	/* release the blkcg and parent blkg refs this blkg has been holding */
+	css_put(&blkg->blkcg->css);
+	if (blkg->parent)
+		blkg_put(blkg->parent);
+
+	wb_congested_put(blkg->wb_congested);
+
+	blkg_free(blkg);
+}
+EXPORT_SYMBOL_GPL(__blkg_release_rcu);
+
+/*
  * The next function used by blk_queue_for_each_rl().  It's a bit tricky
  * because the root blkg uses @q->root_rl instead of its own rl.
  */
@@ -1796,7 +1748,8 @@ void blkcg_maybe_throttle_current(void)
 	blkg = blkg_lookup(blkcg, q);
 	if (!blkg)
 		goto out;
-	if (!blkg_tryget(blkg))
+	blkg = blkg_try_get(blkg);
+	if (!blkg)
 		goto out;
 	rcu_read_unlock();