Commit a20135ff authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe

writeback: don't drain bdi_writeback_congested on bdi destruction

52ebea74 ("writeback: make backing_dev_info host cgroup-specific
bdi_writebacks") made bdi (backing_dev_info) host per-cgroup wb's
(bdi_writeback's).  As the congested state needs to be per-wb and
referenced from blkcg side and multiple wbs, the patch made all
non-root cong's (bdi_writeback_congested's) reference counted and
indexed on bdi.

When a bdi is destroyed, cgwb_bdi_destroy() tries to drain all
non-root cong's; however, this can hang indefinitely because wb's can
also be referenced from blkcg_gq's which are destroyed after bdi
destruction is complete.

This patch fixes the bug by updating bdi destruction to not wait for
cong's to drain.  A cong is unlinked from bdi->cgwb_congested_tree on
bdi destuction regardless of its reference count as the bdi may go
away any point after destruction.  wb_congested_put() checks whether
the cong is already unlinked on release.
Signed-off-by: default avatarTejun Heo <>
Reported-by: default avatarJon Christopherson <>
Fixes: 52ebea74 ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
Tested-by: default avatarJon Christopherson <>
Signed-off-by: default avatarJens Axboe <>
parent a13f35e8
......@@ -425,7 +425,6 @@ retry:
new_congested = NULL;
rb_link_node(&congested->rb_node, parent, node);
rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
goto found;
......@@ -456,7 +455,6 @@ found:
void wb_congested_put(struct bdi_writeback_congested *congested)
struct backing_dev_info *bdi = congested->bdi;
unsigned long flags;
......@@ -465,12 +463,15 @@ void wb_congested_put(struct bdi_writeback_congested *congested)
rb_erase(&congested->rb_node, &congested->bdi->cgwb_congested_tree);
/* bdi might already have been destroyed leaving @congested unlinked */
if (congested->bdi) {
congested->bdi = NULL;
spin_unlock_irqrestore(&cgwb_lock, flags);
if (atomic_dec_and_test(&bdi->usage_cnt))
static void cgwb_release_workfn(struct work_struct *work)
......@@ -675,13 +676,22 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
struct radix_tree_iter iter;
struct bdi_writeback_congested *congested, *congested_n;
void **slot;
WARN_ON(test_bit(WB_registered, &bdi->wb.state));
radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
rbtree_postorder_for_each_entry_safe(congested, congested_n,
&bdi->cgwb_congested_tree, rb_node) {
rb_erase(&congested->rb_node, &bdi->cgwb_congested_tree);
congested->bdi = NULL; /* mark @congested unlinked */
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