Btrfs: avoid waiting for delayed refs when we must not (8ca78f3e) · Commits · 戴 / test

We track two conditions to decide if we should sleep while waiting for more delayed refs, the number of delayed refs (num_refs) and the first entry in the list of blockers (first_seq). When we suspect staleness, we save num_refs and do one more cycle. If nothing changes, we then save first_seq for later comparison and do wait_event. We ought to save first_seq the very same moment we're saving num_refs. Otherwise we cannot be sure that nothing has changed and we might start waiting when we shouldn't, which could lead to starvation. Signed-off-by:

Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/extent-tree.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -2347,12 +2347,10 @@ next:
		return count;
		}


		static void wait_for_more_refs(struct btrfs_delayed_ref_root *delayed_refs,
		unsigned long num_refs)
		unsigned long num_refs,
		struct list_head *first_seq)
		{
		struct list_head *first_seq = delayed_refs->seq_head.next;

		spin_unlock(&delayed_refs->lock);
		pr_debug("waiting for more refs (num %ld, first %p)\n",
		num_refs, first_seq);
		@@ -2381,6 +2379,7 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
		struct btrfs_delayed_ref_root *delayed_refs;
		struct btrfs_delayed_ref_node *ref;
		struct list_head cluster;
		struct list_head *first_seq = NULL;
		int ret;
		u64 delayed_start;
		int run_all = count == (unsigned long)-1;
		@@ -2436,8 +2435,10 @@ again:
		*/
		consider_waiting = 1;
		num_refs = delayed_refs->num_entries;
		first_seq = root->fs_info->tree_mod_seq_list.next;
		} else {
		wait_for_more_refs(delayed_refs, num_refs);
		wait_for_more_refs(delayed_refs,
		num_refs, first_seq);
		/*
		* after waiting, things have changed. we
		* dropped the lock and someone else might have

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