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, 45 insertions, 100 deletions

diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 1e76ceebeb5d..6d766a19f2bb 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -126,7 +126,7 @@ struct blkcg_gq {
 	struct request_list		rl;
 
 	/* reference count */
-	struct percpu_ref		refcnt;
+	atomic_t			refcnt;
 
 	/* is this blkg online? protected by both blkcg and q locks */
 	bool				online;
@@ -184,8 +184,6 @@ extern struct cgroup_subsys_state * const blkcg_root_css;
 
 struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,
 				      struct request_queue *q, bool update_hint);
-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);
 int blkcg_init_queue(struct request_queue *q);
@@ -232,59 +230,22 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		   char *input, struct blkg_conf_ctx *ctx);
 void blkg_conf_finish(struct blkg_conf_ctx *ctx);
 
-/**
- * blkcg_css - find the current css
- *
- * Find the css associated with either the kthread or the current task.
- * This may return a dying css, so it is up to the caller to use tryget logic
- * to confirm it is alive and well.
- */
-static inline struct cgroup_subsys_state *blkcg_css(void)
-{
-	struct cgroup_subsys_state *css;
-
-	css = kthread_blkcg();
-	if (css)
-		return css;
-	return task_css(current, io_cgrp_id);
-}
 
 static inline struct blkcg *css_to_blkcg(struct cgroup_subsys_state *css)
 {
 	return css ? container_of(css, struct blkcg, css) : NULL;
 }
 
-/**
- * __bio_blkcg - internal version of bio_blkcg for bfq and cfq
- *
- * DO NOT USE.
- * There is a flaw using this version of the function.  In particular, this was
- * used in a broken paradigm where association was called on the given css.  It
- * is possible though that the returned css from task_css() is in the process
- * of dying due to migration of the current task.  So it is improper to assume
- * *_get() is going to succeed.  Both BFQ and CFQ rely on this logic and will
- * take additional work to handle more gracefully.
- */
-static inline struct blkcg *__bio_blkcg(struct bio *bio)
-{
-	if (bio && bio->bi_blkg)
-		return bio->bi_blkg->blkcg;
-	return css_to_blkcg(blkcg_css());
-}
-
-/**
- * bio_blkcg - grab the blkcg associated with a bio
- * @bio: target bio
- *
- * This returns the blkcg associated with a bio, NULL if not associated.
- * Callers are expected to either handle NULL or know association has been
- * done prior to calling this.
- */
 static inline struct blkcg *bio_blkcg(struct bio *bio)
 {
-	if (bio && bio->bi_blkg)
-		return bio->bi_blkg->blkcg;
-	return NULL;
+	struct cgroup_subsys_state *css;
+
+	if (bio && bio->bi_css)
+		return css_to_blkcg(bio->bi_css);
+	css = kthread_blkcg();
+	if (css)
+		return css_to_blkcg(css);
+	return css_to_blkcg(task_css(current, io_cgrp_id));
 }
 
 static inline bool blk_cgroup_congested(void)
@@ -490,35 +451,26 @@ static inline int blkg_path(struct blkcg_gq *blkg, char *buf, int buflen)
  */
 static inline void blkg_get(struct blkcg_gq *blkg)
 {
-	percpu_ref_get(&blkg->refcnt);
+	WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
+	atomic_inc(&blkg->refcnt);
 }
 
 /**
- * blkg_tryget - try and get a blkg reference
+ * blkg_try_get - try and get a blkg reference
  * @blkg: blkg to get
  *
  * This is for use when doing an RCU lookup of the blkg.  We may be in the midst
  * of freeing this blkg, so we can only use it if the refcnt is not zero.
  */
-static inline bool blkg_tryget(struct blkcg_gq *blkg)
+static inline struct blkcg_gq *blkg_try_get(struct blkcg_gq *blkg)
 {
-	return percpu_ref_tryget(&blkg->refcnt);
+	if (atomic_inc_not_zero(&blkg->refcnt))
+		return blkg;
+	return NULL;
 }
 
-/**
- * blkg_tryget_closest - try and get a blkg ref on the closet blkg
- * @blkg: blkg to get
- *
- * This walks up the blkg tree to find the closest non-dying blkg and returns
- * the blkg that it did association with as it may not be the passed in blkg.
- */
-static inline struct blkcg_gq *blkg_tryget_closest(struct blkcg_gq *blkg)
-{
-	while (!percpu_ref_tryget(&blkg->refcnt))
-		blkg = blkg->parent;
 
-	return blkg;
-}
+void __blkg_release_rcu(struct rcu_head *rcu);
 
 /**
  * blkg_put - put a blkg reference
@@ -526,7 +478,9 @@ static inline struct blkcg_gq *blkg_tryget_closest(struct blkcg_gq *blkg)
  */
 static inline void blkg_put(struct blkcg_gq *blkg)
 {
-	percpu_ref_put(&blkg->refcnt);
+	WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
+	if (atomic_dec_and_test(&blkg->refcnt))
+		call_rcu(&blkg->rcu_head, __blkg_release_rcu);
 }
 
 /**
@@ -579,36 +533,25 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
 
 	rcu_read_lock();
 
-	if (bio && bio->bi_blkg) {
-		blkcg = bio->bi_blkg->blkcg;
-		if (blkcg == &blkcg_root)
-			goto rl_use_root;
-
-		blkg_get(bio->bi_blkg);
-		rcu_read_unlock();
-		return &bio->bi_blkg->rl;
-	}
+	blkcg = bio_blkcg(bio);
 
-	blkcg = css_to_blkcg(blkcg_css());
+	/* bypass blkg lookup and use @q->root_rl directly for root */
 	if (blkcg == &blkcg_root)
-		goto rl_use_root;
+		goto root_rl;
 
+	/*
+	 * Try to use blkg->rl.  blkg lookup may fail under memory pressure
+	 * or if either the blkcg or queue is going away.  Fall back to
+	 * root_rl in such cases.
+	 */
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg))
-		blkg = __blkg_lookup_create(blkcg, q);
-
-	if (blkg->blkcg == &blkcg_root || !blkg_tryget(blkg))
-		goto rl_use_root;
+		goto root_rl;
 
+	blkg_get(blkg);
 	rcu_read_unlock();
 	return &blkg->rl;
-
-	/*
-	 * Each blkg has its own request_list, however, the root blkcg
-	 * uses the request_queue's root_rl.  This is to avoid most
-	 * overhead for the root blkcg.
-	 */
-rl_use_root:
+root_rl:
 	rcu_read_unlock();
 	return &q->root_rl;
 }
@@ -854,26 +797,32 @@ static inline bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg
 				  struct bio *bio) { return false; }
 #endif
 
-
-static inline void blkcg_bio_issue_init(struct bio *bio)
-{
-	bio_issue_init(&bio->bi_issue, bio_sectors(bio));
-}
-
 static inline bool blkcg_bio_issue_check(struct request_queue *q,
 					 struct bio *bio)
 {
+	struct blkcg *blkcg;
 	struct blkcg_gq *blkg;
 	bool throtl = false;
 
 	rcu_read_lock();
+	blkcg = bio_blkcg(bio);
+
+	/* associate blkcg if bio hasn't attached one */
+	bio_associate_blkcg(bio, &blkcg->css);
 
-	bio_associate_create_blkg(q, bio);
-	blkg = bio->bi_blkg;
+	blkg = blkg_lookup(blkcg, q);
+	if (unlikely(!blkg)) {
+		spin_lock_irq(q->queue_lock);
+		blkg = blkg_lookup_create(blkcg, q);
+		if (IS_ERR(blkg))
+			blkg = NULL;
+		spin_unlock_irq(q->queue_lock);
+	}
 
 	throtl = blk_throtl_bio(q, blkg, bio);
 
 	if (!throtl) {
+		blkg = blkg ?: q->root_blkg;
 		/*
 		 * If the bio is flagged with BIO_QUEUE_ENTERED it means this
 		 * is a split bio and we would have already accounted for the
@@ -885,8 +834,6 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 		blkg_rwstat_add(&blkg->stat_ios, bio->bi_opf, 1);
 	}
 
-	blkcg_bio_issue_init(bio);
-
 	rcu_read_unlock();
 	return !throtl;
 }
@@ -983,7 +930,6 @@ static inline int blkcg_activate_policy(struct request_queue *q,
 static inline void blkcg_deactivate_policy(struct request_queue *q,
 					   const struct blkcg_policy *pol) { }
 
-static inline struct blkcg *__bio_blkcg(struct bio *bio) { return NULL; }
 static inline struct blkcg *bio_blkcg(struct bio *bio) { return NULL; }
 
 static inline struct blkg_policy_data *blkg_to_pd(struct blkcg_gq *blkg,
@@ -999,7 +945,6 @@ static inline void blk_put_rl(struct request_list *rl) { }
 static inline void blk_rq_set_rl(struct request *rq, struct request_list *rl) { }
 static inline struct request_list *blk_rq_rl(struct request *rq) { return &rq->q->root_rl; }
 
-static inline void blkcg_bio_issue_init(struct bio *bio) { }
 static inline bool blkcg_bio_issue_check(struct request_queue *q,
 					 struct bio *bio) { return true; }