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Commit a20135ff authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe
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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 <tj@kernel.org>
Reported-by: default avatarJon Christopherson <jon@jons.org>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=100681


Fixes: 52ebea74 ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
Tested-by: default avatarJon Christopherson <jon@jons.org>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent a13f35e8

mm/backing-dev.c

+16 −6
Original line number Diff line number Diff line
@@ -425,7 +425,6 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp) 
		new_congested = NULL;

		rb_link_node(&congested->rb_node, parent, node);

		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);

		atomic_inc(&bdi->usage_cnt);

		goto found;

	}
@@ -456,7 +455,6 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp) 
 */

void wb_congested_put(struct bdi_writeback_congested *congested)

{

	struct backing_dev_info *bdi = congested->bdi;

	unsigned long flags;



	local_irq_save(flags);
@@ -465,12 +463,15 @@ void wb_congested_put(struct bdi_writeback_congested *congested) 
		return;

	}



	rb_erase(&congested->rb_node, &congested->bdi->cgwb_congested_tree);

	/* bdi might already have been destroyed leaving @congested unlinked */

	if (congested->bdi) {

		rb_erase(&congested->rb_node,

			 &congested->bdi->cgwb_congested_tree);

		congested->bdi = NULL;

	}



	spin_unlock_irqrestore(&cgwb_lock, flags);

	kfree(congested);



	if (atomic_dec_and_test(&bdi->usage_cnt))

		wake_up_all(&cgwb_release_wait);

}



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));



	spin_lock_irq(&cgwb_lock);



	radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)

		cgwb_kill(*slot);



	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 */

	}



	spin_unlock_irq(&cgwb_lock);



	/*