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Subject[PATCH block/for-next 3/4] cgroup, block: implement task_get_css() and use it in bio_associate_current()
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@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Vivek Goyal <vgoyal@redhat.com>
---
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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