blkcg: remove additional reference to the css

The previous patch in this series removed carrying around a pointer to
the css in blkg. However, the blkg association logic still relied on
taking a reference on the css to ensure we wouldn't fail in getting a
reference for the blkg.

Here the implicit dependency on the css is removed. The association
continues to rely on the tryget logic walking up the blkg tree. This
streamlines the three ways that association can happen: normal, swap,
and writeback.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 4 insertions, 48 deletions

diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index c41cfcc2b4d8..2951ea3541b1 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -249,47 +249,6 @@ static inline struct cgroup_subsys_state *blkcg_css(void)
         return task_css(current, io_cgrp_id);
 }
 
-/**
- * blkcg_get_css - find and get a reference to the css
- *
- * Find the css associated with either the kthread or the current task.
- * This takes a reference on the blkcg which will need to be managed by the
- * caller.
- */
-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 {
-                /*
-                 * This is a bit complicated.  It is possible task_css is seeing
-                 * an old css pointer here.  This is caused by the current
-                 * thread migrating away from this cgroup and this cgroup dying.
-                 * css_tryget() will fail when trying to take a ref on a cgroup
-                 * that's ref count has hit 0.
-                 *
-                 * Therefore, if it does fail, this means current must have
-                 * been swapped away already and this is waiting for it to
-                 * propagate on the polling cpu.  Hence the use of cpu_relax().
-                 */
-                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;
@@ -628,10 +587,8 @@ 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());
+        if (!blkcg)
+                blkcg = css_to_blkcg(blkcg_css());
 
         /* bypass blkg lookup and use @q->root_rl directly for root */
         if (blkcg == &blkcg_root)
@@ -646,7 +603,8 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
         if (unlikely(!blkg))
                 goto root_rl;
 
-        blkg_get(blkg);
+        if (!blkg_try_get(blkg))
+                goto root_rl;
         rcu_read_unlock();
         return &blkg->rl;
 root_rl:
@@ -663,8 +621,6 @@ root_rl:
  */
 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);
 }