writeback: don't drain bdi_writeback_congested on bdi destruction (a20135ff) · Commits · 戴 / test

("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:

Tejun Heo <tj@kernel.org> Reported-by:

Jon Christopherson <jon@jons.org> Link: https://bugzilla.kernel.org/show_bug.cgi?id=100681 Fixes: ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks") Tested-by:

Jon Christopherson <jon@jons.org> Signed-off-by:

Jens Axboe <axboe@fb.com>

mm/backing-dev.c

+16 −6

Original line number	Diff line number	Diff line
		@@ -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);
		atomic_inc(&bdi->usage_cnt);
		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;

		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);

		/*

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