Re: [PATCH v2 3/3] blk-cgroup: move async bio punt state to blkcg

Hi,

在 2026/9/13 21:29, Nilay Shroff 写道:

On 9/13/26 12:24 PM, Yu Kuai wrote:

quoted

From: Yu Kuai <yukuai@fygo.io>

blkcg_punt_bio_submit() currently queues punted bios on 
blkg->async_bios,
so it has to call bio_blkg() to find or create a queue-local blkg.  Bios
now carry and pin the blkcg css, so punted bio lifetime no longer 
needs to
be anchored by a blkg.

Keeping the punt state in blkg can instantiate a blkg even when no blkcg
policy is enabled, just to bounce submission from a shared kthread.  
Move
async_bio_lock, async_bios and async_bio_work to struct blkcg, and queue
punted bios on bio_blkcg() for non-root cgroups.  Root or 
unassociated bios
are submitted directly.

This preserves the priority-inversion avoidance while preventing
blkcg_punt_bio_submit() from creating blkgs that are not needed by any
policy.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
  block/blk-cgroup.c | 52 ++++++++++++++++++++++++++--------------------
  block/blk-cgroup.h | 14 ++++++-------
  2 files changed, 35 insertions(+), 31 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 59ccfefe16a8..aa3cee107ebe 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c

@@ -180,14 +180,10 @@ static void blkg_free(struct blkcg_gq *blkg)

    static void __blkg_release(struct rcu_head *rcu)
  {
      struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, 
rcu_head);
  -#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
-    WARN_ON(!bio_list_empty(&blkg->async_bios));
-#endif
-
      blkg_free(blkg);
  }
    /*
   * A group is RCU protected, but having an rcu lock does not mean 
that one

@@ -226,23 +222,23 @@ static void blkg_release(struct percpu_ref *ref)

  }
    #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
  static struct workqueue_struct *blkcg_punt_bio_wq;
  -static void blkg_async_bio_workfn(struct work_struct *work)
+static void blkcg_async_bio_workfn(struct work_struct *work)
  {
-    struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
-                         async_bio_work);
+    struct blkcg *blkcg = container_of(work, struct blkcg, 
async_bio_work);
      struct bio_list bios = BIO_EMPTY_LIST;
      struct bio *bio;
      struct blk_plug plug;
      bool need_plug = false;
  -    /* as long as there are pending bios, @blkg can't go away */
-    spin_lock(&blkg->async_bio_lock);
-    bio_list_merge_init(&bios, &blkg->async_bios);
-    spin_unlock(&blkg->async_bio_lock);
+    /* as long as there are pending bios, @blkcg can't go away */
+    {
+        guard(spinlock)(&blkcg->async_bio_lock);
+        bio_list_merge_init(&bios, &blkcg->async_bios);
+    }

Instead of using guard(spinlock)(...) here, I think we could use the
simpler spin_lock()/spin_unlock() helpers. IMO, they are easier
to read and reason about for these short critical sections.

Ok.

quoted

      /* start plug only when bio_list contains at least 2 bios */
      if (bios.head && bios.head->bi_next) {
          need_plug = true;
          blk_start_plug(&plug);

@@ -259,19 +255,20 @@ static void blkg_async_bio_workfn(struct 

work_struct *work)
   * cgroup.  Use this helper instead of submit_bio to punt the 
actual issuing to
   * a dedicated per-blkcg work item to avoid such priority inversions.
   */
  void blkcg_punt_bio_submit(struct bio *bio)
  {
-    struct blkcg_gq *blkg = bio_blkg(bio);
+    struct blkcg *blkcg = bio_blkcg(bio);
  -    if (blkg && blkg->parent) {
-        spin_lock(&blkg->async_bio_lock);
-        bio_list_add(&blkg->async_bios, bio);
-        spin_unlock(&blkg->async_bio_lock);
-        queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
+    if (blkcg && cgroup_parent(blkcg->css.cgroup)) {
+        {
+            guard(spinlock)(&blkcg->async_bio_lock);
+            bio_list_add(&blkcg->async_bios, bio);
+        }
+        queue_work(blkcg_punt_bio_wq, &blkcg->async_bio_work);

Again same here, replace guard() with spin_lock() and spin_unlock()
helpers.

quoted

      } else {
-        /* Never bounce if there is no non-root blkg to queue on. */
+        /* Never bounce if there is no non-root blkcg to queue on. */
          submit_bio(bio);
      }
  }
  EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);
  @@ -350,15 +347,10 @@ static struct blkcg_gq *blkg_alloc(struct 
blkcg *blkcg, struct gendisk *disk,
      blkg->q = disk->queue;
      INIT_LIST_HEAD(&blkg->q_node);
      blkg->blkcg = blkcg;
      blkg->blkcg_id = blkcg->css.id;
      blkg->iostat.blkg = blkg;
-#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
-    spin_lock_init(&blkg->async_bio_lock);
-    bio_list_init(&blkg->async_bios);
-    INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
-#endif
        u64_stats_init(&blkg->iostat.sync);
      for_each_possible_cpu(cpu) {
          u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
          per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;

@@ -1399,10 +1391,16 @@ static void blkcg_css_free(struct 

cgroup_subsys_state *css)
          if (blkcg->cpd[i])
              blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);
        mutex_unlock(&blkcg_pol_mutex);
  +#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+    {
+        guard(spinlock)(&blkcg->async_bio_lock);
+        WARN_ON(!bio_list_empty(&blkcg->async_bios));
+    }
+#endif

This is a slightly different case. At this point blkcg_css_free() is
freeing the blkcg object after its final reference has gone away, so
there should be no concurrent context accessing blkcg->async_bios.
Therefore, I don't think we need to acquire async_bio_lock here just
to perform the WARN_ON() check.

Perhaps is it better just to remove the check? blkcg will be pinned
by any bio inside the list, so I believe this is safe.

The clang context annotation cannot infer this object-lifetime property
and will therefore report an unprotected access. I think we should
explicitly mark this access as context-unsafe.

But wait, even better, we could introduce a bio_list_empty_careful() 
helper,
similar to list_empty_careful(), for this purpose:

static inline bool bio_list_empty_careful(const struct bio_list *bl)
    __context_unsafe(/* intentional lockless access to @bl->head */)
{
    return bl->head == NULL;
}

Then this could simply become:

WARN_ON(!bio_list_empty_careful(&blkcg->async_bios));

quoted

      free_percpu(blkcg->lhead);
      kfree(blkcg);
  }
    static struct cgroup_subsys_state *

@@ -1447,10 +1445,18 @@ blkcg_css_alloc(struct cgroup_subsys_state 

*parent_css)
      }
        spin_lock_init(&blkcg->lock);
      refcount_set(&blkcg->online_pin, 1);
      INIT_HLIST_HEAD(&blkcg->blkg_list);
+#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
+    spin_lock_init(&blkcg->async_bio_lock);
+    {
+        guard(spinlock)(&blkcg->async_bio_lock);
+        bio_list_init(&blkcg->async_bios);
+    }
+    INIT_WORK(&blkcg->async_bio_work, blkcg_async_bio_workfn);
+#endif

This is interesting. As you know, while an object is being allocated
and before it is published, it can't be accessed concurrently. So
guarding blkcg->async_bios with blkcg->async_bio_lock is not necessary
here. Moreover, since blkcg is zero-initialized, we could simply remove
both the guard(...) and the bio_list_init() call above.

Thanks,
--Nilay

-- 
Thanks,
Kuai