[PATCH 04/15] blkcg: fix ref count issue with bio_blkcg using task_css

[Dennis Zhou <dennisszhou@gmail.com>]

From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>

The accessor function bio_blkcg either returns the blkcg associated with
the bio or finds one in the current context. This can cause an issue
when trying to associate a bio with a blkcg. Particularly, it's the
third case that is problematic:

	return css_to_blkcg(task_css(current, io_cgrp_id));

As the above may race against task migration and the cgroup exiting, it
is not always ok to take a reference on the blkcg returned from
bio_blkcg.

This patch adds association ahead of calling bio_blkcg rather than
after. This prevents makes association a required and explicit step
along the code paths for calling bio_blkcg. blk_get_rl is modified
as well to get a reference to the blkcg it may use and blk_put_rl
will always put the reference back. Association is also moved above the
bio_blkcg call to ensure it will not return NULL in blk-iolatency.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
---
 block/bio.c                | 10 +++++--
 block/blk-iolatency.c      |  2 +-
 include/linux/blk-cgroup.h | 53 ++++++++++++++++++++++++++++++++------
 3 files changed, 54 insertions(+), 11 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 4473ccd22987..09a31e4d46bb 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1962,13 +1962,19 @@ int bio_associate_blkcg_from_page(struct bio *bio, struct page *page)
  *
  * This function takes an extra reference of @blkcg_css which will be put
  * when @bio is released.  The caller must own @bio and is responsible for
- * synchronizing calls to this function.
+ * synchronizing calls to this function.  If @blkcg_css is NULL, a call to
+ * blkcg_get_css finds the current css from the kthread or task.
  */
 int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css)
 {
 	if (unlikely(bio->bi_css))
 		return -EBUSY;
-	css_get(blkcg_css);
+
+	if (blkcg_css)
+		css_get(blkcg_css);
+	else
+		blkcg_css = blkcg_get_css();
+
 	bio->bi_css = blkcg_css;
 	return 0;
 }
diff --git a/block/blk-iolatency.c b/block/blk-iolatency.c
index 19923f8a029d..62fdd9002c29 100644
--- a/block/blk-iolatency.c
+++ b/block/blk-iolatency.c
@@ -404,8 +404,8 @@ static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio,
 		return;
 
 	rcu_read_lock();
+	bio_associate_blkcg(bio, NULL);
 	blkcg = bio_blkcg(bio);
-	bio_associate_blkcg(bio, &blkcg->css);
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg)) {
 		if (!lock)
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index c7386464ec4c..d3cafb1eda48 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -230,22 +230,52 @@ 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_get_css - find and get a reference to the css
+ *
+ * Find the css associated with either the kthread or the current task.
+ */
+static inline struct cgroup_subsys_state *blkcg_get_css(void)
+{
+	struct cgroup_subsys_state *css;
+
+	rcu_read_lock();
+
+	css = kthread_blkcg();
+	if (css) {
+		css_get(css);
+	} else {
+		while (true) {
+			css = task_css(current, io_cgrp_id);
+			if (likely(css_tryget(css)))
+				break;
+			cpu_relax();
+		}
+	}
+
+	rcu_read_unlock();
+
+	return css;
+}
 
 static inline struct blkcg *css_to_blkcg(struct cgroup_subsys_state *css)
 {
 	return css ? container_of(css, struct blkcg, css) : NULL;
 }
 
+/**
+ * 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)
 {
-	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));
+	return NULL;
 }
 
 static inline bool blk_cgroup_congested(void)
@@ -519,6 +549,11 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
 	rcu_read_lock();
 
 	blkcg = bio_blkcg(bio);
+	if (blkcg) {
+		css_get(&blkcg->css);
+	} else {
+		blkcg = css_to_blkcg(blkcg_get_css());
+	}
 
 	/* bypass blkg lookup and use @q->root_rl directly for root */
 	if (blkcg == &blkcg_root)
@@ -550,6 +585,8 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
  */
 static inline void blk_put_rl(struct request_list *rl)
 {
+	/* an additional ref is always taken for rl */
+	css_put(&rl->blkg->blkcg->css);
 	if (rl->blkg->blkcg != &blkcg_root)
 		blkg_put(rl->blkg);
 }
@@ -790,10 +827,10 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	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_blkcg(bio, NULL);
+	blkcg = bio_blkcg(bio);
 
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg)) {
-- 
2.17.1