[PATCH block/for-next 3/4] cgroup, block: implement task_get_css() and use it in bio_associate_current()

[Tejun Heo]

bio_associate_current() currently open codes task_css() and
css_tryget_online() to find and pin $current's blkcg css.  Abstract it
into task_get_css() which is implemented from cgroup side.  As a task
is always associated with an online css for every subsystem except
while the css_set update is propagating, task_get_css() retries till
css_tryget_online() succeeds.

This is a cleanup and shouldn't lead to noticeable behavior changes.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
---
Hello,

This one can be routed through either cgroup or block tree but I think
it'd be easier to route this through block as there will be more block
changes in the area.

Thanks.

 block/bio.c            |   11 +----------
 include/linux/cgroup.h |   25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 10 deletions(-)

--- a/block/bio.c
+++ b/block/bio.c
@@ -1997,7 +1997,6 @@ EXPORT_SYMBOL(bioset_create_nobvec);
 int bio_associate_current(struct bio *bio)
 {
 	struct io_context *ioc;
-	struct cgroup_subsys_state *css;
 
 	if (bio->bi_ioc)
 		return -EBUSY;
@@ -2006,17 +2005,9 @@ int bio_associate_current(struct bio *bi
 	if (!ioc)
 		return -ENOENT;
 
-	/* acquire active ref on @ioc and associate */
 	get_io_context_active(ioc);
 	bio->bi_ioc = ioc;
-
-	/* associate blkcg if exists */
-	rcu_read_lock();
-	css = task_css(current, blkio_cgrp_id);
-	if (css && css_tryget_online(css))
-		bio->bi_css = css;
-	rcu_read_unlock();
-
+	bio->bi_css = task_get_css(current, blkio_cgrp_id);
 	return 0;
 }
 
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -748,6 +748,31 @@ static inline struct cgroup_subsys_state
 }
 
 /**
+ * task_get_css - find and get the css for (task, subsys)
+ * @task: the target task
+ * @subsys_id: the target subsystem ID
+ *
+ * Find the css for the (@task, @subsys_id) combination, increment a
+ * reference on and return it.  This function is guaranteed to return a
+ * valid css.
+ */
+static inline struct cgroup_subsys_state *
+task_get_css(struct task_struct *task, int subsys_id)
+{
+	struct cgroup_subsys_state *css;
+
+	rcu_read_lock();
+	while (true) {
+		css = task_css(task, subsys_id);
+		if (likely(css_tryget_online(css)))
+			break;
+		cpu_relax();
+	}
+	rcu_read_unlock();
+	return css;
+}
+
+/**
  * task_css_is_root - test whether a task belongs to the root css
  * @task: the target task
  * @subsys_id: the target subsystem ID
--
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