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Btrfs: avoid waiting for delayed refs when we must not

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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>
This commit is contained in:
Jan Schmidt 2012-06-27 15:05:48 +02:00
parent cb77fcd885
commit 8ca78f3eda
1 changed files with 6 additions and 5 deletions
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11
fs/btrfs/extent-tree.c
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@ -2347,12 +2347,10 @@ static noinline int run_clustered_refs(struct btrfs_trans_handle *trans,
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 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
*/
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