Ensure cgwb (cgroup writeback) structures are uniquely associated with a
memcg-blkcg pair to prevent inconsistencies when concurrent cgwb_create
calls race. This resolves a scenario where two threads creating cgwbs
for the same memory cgroup (memcg) but different I/O control groups (blkcg)
could insert conflicting entries.
The fix rechecks for existing cgwbs under the cgwb_lock spinlock after
initial creation. If a conflicting cgwb (same memcg, different blkcg) is
found, it is killed before inserting the new entry. This guarantees a
1:1 relationship between memcg-blkcg pairs and their cgwbs, preserving
system invariants.
Signed-off-by: sooraj <sooraj20636@xxxxxxxxx>
---
mm/backing-dev.c | 43 +++++++++++++++++++++++++++++--------------
1 file changed, 29 insertions(+), 14 deletions(-)
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index e61bbb1bd622..67acb565e9a7 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -723,24 +723,39 @@ static int cgwb_create(struct backing_dev_info *bdi,
spin_lock_irqsave(&cgwb_lock, flags);
if (test_bit(WB_registered, &bdi->wb.state) &&
blkcg_cgwb_list->next && memcg_cgwb_list->next) {
- /* we might have raced another instance of this function */
- ret = radix_tree_insert(&bdi->cgwb_tree, memcg_css->id, wb);
- if (!ret) {
- list_add_tail_rcu(&wb->bdi_node, &bdi->wb_list);
- list_add(&wb->memcg_node, memcg_cgwb_list);
- list_add(&wb->blkcg_node, blkcg_cgwb_list);
- blkcg_pin_online(blkcg_css);
- css_get(memcg_css);
- css_get(blkcg_css);
+ /* Re-check under lock to handle races */
+ struct bdi_writeback *existing;
+
+ existing = radix_tree_lookup(&bdi->cgwb_tree, memcg_css->id);
+ if (existing) {
+ if (existing->blkcg_css != blkcg_css) {
+ cgwb_kill(existing);
+ existing = NULL;
+ } else {
+ ret = 0; /* Already exists, treat as success */
+ }
+ }
+
+ if (!existing) {
+ ret = radix_tree_insert(&bdi->cgwb_tree, memcg_css->id, wb);
+ if (!ret) {
+ list_add_tail_rcu(&wb->bdi_node, &bdi->wb_list);
+ list_add(&wb->memcg_node, memcg_cgwb_list);
+ list_add(&wb->blkcg_node, blkcg_cgwb_list);
+ blkcg_pin_online(blkcg_css);
+ css_get(memcg_css);
+ css_get(blkcg_css);
+ }
}
}
spin_unlock_irqrestore(&cgwb_lock, flags);
- if (ret) {
- if (ret == -EEXIST)
- ret = 0;
+
+ if (!ret)
+ goto out_put;
+ if (ret == -EEXIST)
+ ret = 0; /* Lost race, another thread created the same wb */
+ else
goto err_fprop_exit;
- }
- goto out_put;
err_fprop_exit:
bdi_put(bdi);
--
2.45.2
