Btrfs: avoid waiting for delayed refs when we must not
Jan Schmidt [Wed, 27 Jun 2012 13:05:48 +0000 (15:05 +0200)]
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

index 4b5a1e1..6e1d367 100644 (file)
@@ -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