[RFC PATCH v2 1/4] blk-cgroup: wait for old blkgs to leave queue before disk rebind
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From: Yu Kuai <yukuai@kernel.org>
Date: 2026-08-11 06:50:38
Also in:
cgroups, dm-devel, gfs2, linux-bcache, linux-block, linux-doc, linux-fsdevel, linux-mm, lkml, nvdimm, virtualization
Subsystem:
block layer, control group - block io controller (blkio), the rest · Maintainers:
Jens Axboe, Tejun Heo, Josef Bacik, Linus Torvalds
Revision v2 of 7 in this series.
Revisions (7)
From: Yu Kuai <yukuai@fygo.io>
blkcg_init_disk() currently waits for q->root_blkg to become NULL before
initializing blkcg state for a rebound disk. However, blkg_destroy_all()
clears q->root_blkg after calling blkg_destroy() for each blkg. At that
point the initial references have only been killed, and the blkgs remain
on q->blkg_list until the remaining references drain and
blkg_free_workfn() removes them.
A rebound disk can therefore install new blkcg state while old blkgs are
still attached to the request queue. Wait for q->blkg_list to become empty
instead, and wake the waiter when the final blkg is removed. This covers
the complete queue-side blkg lifetime without adding separate state.
Fixes: 3dbaacf6ab68 ("blk-cgroup: wait for blkcg cleanup before initializing new disk")
Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
block/blk-cgroup.c | 14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 354637f3b158..229348273437 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c@@ -131,10 +131,12 @@ static void blkg_free_workfn(struct work_struct *work) blkcg_policy[i]->pd_free_fn(blkg->pd[i]); if (blkg->parent) blkg_put(blkg->parent); spin_lock_irq(&q->queue_lock); list_del_init(&blkg->q_node); + if (list_empty(&q->blkg_list)) + wake_up_var(&q->blkg_list); spin_unlock_irq(&q->queue_lock); mutex_unlock(&q->blkcg_mutex); /* * Release blkcg css ref only after blkg is removed from q->blkg_list,
@@ -607,12 +609,10 @@ static void blkg_destroy_all(struct gendisk *disk) } q->root_blkg = NULL; spin_unlock_irq(&q->queue_lock); mutex_unlock(&q->blkcg_mutex); - - wake_up_var(&q->root_blkg); } static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src) { int i;
@@ -1457,18 +1457,14 @@ int blkcg_init_disk(struct gendisk *disk) bool preloaded; /* * If the queue is shared across disk rebind (e.g., SCSI), the * previous disk's blkcg state is cleaned up asynchronously via - * disk_release() -> blkcg_exit_disk(). Wait for that cleanup to - * finish (indicated by root_blkg becoming NULL) before setting up - * new blkcg state. Otherwise, we may overwrite q->root_blkg while - * the old one is still alive, and radix_tree_insert() in - * blkg_create() will fail with -EEXIST because the old entries - * still occupy the same queue id slot in blkcg->blkg_tree. + * disk_release() -> blkcg_exit_disk(). Wait for all old blkgs to be + * removed from the queue list before setting up new blkcg state. */ - wait_var_event(&q->root_blkg, !READ_ONCE(q->root_blkg)); + wait_var_event(&q->blkg_list, list_empty_careful(&q->blkg_list)); new_blkg = blkg_alloc(&blkcg_root, disk, GFP_KERNEL); if (!new_blkg) return -ENOMEM;
--
2.51.0