[PATCH v4] blk-cgroup: fix leaks and online flag on radix_tree_insert failure

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

When radix_tree_insert() fails in blkg_create(), the error path has two
issues:

1. blkg->online is set to true unconditionally, even when the blkg was
   never fully inserted.  Move the assignment inside the success block.

2. The error path calls blkg_put() without first calling
   percpu_ref_kill().  Because the refcount is still in percpu mode,
   percpu_ref_put() only does this_cpu_sub() without checking for zero,
   so blkg_release() is never triggered.  This permanently leaks the
   blkg memory, its percpu iostat, policy data, the parent blkg
   reference, and the cgroup css reference — the latter preventing the
   cgroup from ever being destroyed.

Fix by replacing blkg_put() with percpu_ref_kill(), matching the pattern
used in blkg_destroy().

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>

---
v4:
- Resend. No functional change. Folded in Acked-by from Tejun Heo and
  rebased onto current mainline (context-only: the err_put_css label was
  renamed to err_free_blkg upstream).

v3:
- Remove the redundant blkg_put() after percpu_ref_kill() to avoid a
  double-put that causes the refcount to go negative and bypass
  blkg_release(), as pointed out by the sashiko AI review.
  v3: https://lore.kernel.org/all/20260507061229.57466-1-cuitao@kylinos.cn/

v2:
- Also fix the percpu_ref leak on the radix_tree_insert() error path by
  adding percpu_ref_kill() before blkg_put(), as pointed out by the
  sashiko AI review.
  v1: https://lore.kernel.org/all/20260506131124.16755-1-cuitao@kylinos.cn/
---
 block/blk-cgroup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d2a1f5903f24..d9676126c5b5 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -434,15 +434,15 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk,
 				blkg->pd[i]->online = true;
 			}
 		}
+		blkg->online = true;
 	}
-	blkg->online = true;
 	spin_unlock(&blkcg->lock);
 
 	if (!ret)
 		return blkg;
 
 	/* @blkg failed fully initialized, use the usual release path */
-	blkg_put(blkg);
+	percpu_ref_kill(&blkg->refcnt);
 	return ERR_PTR(ret);
 
 err_free_blkg:
-- 
2.43.0