On Thu, Nov 22, 2018 at 11:42:28AM +0200, Nikolay Borisov wrote:
>
>
> On 22.11.18 г. 11:12 ч., Nikolay Borisov wrote:
> >
> >
> > On 21.11.18 г. 20:59 ч., Josef Bacik wrote:
> >> From: Josef Bacik <jbacik@xxxxxx>
> >>
> >> We do this dance in cleanup_ref_head and check_ref_cleanup, unify it
> >> into a helper and cleanup the calling functions.
> >>
> >> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> >> Reviewed-by: Omar Sandoval <osandov@xxxxxx>
> >> ---
> >> fs/btrfs/delayed-ref.c | 14 ++++++++++++++
> >> fs/btrfs/delayed-ref.h | 3 ++-
> >> fs/btrfs/extent-tree.c | 22 +++-------------------
> >> 3 files changed, 19 insertions(+), 20 deletions(-)
> >>
> >> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
> >> index 9301b3ad9217..b3e4c9fcb664 100644
> >> --- a/fs/btrfs/delayed-ref.c
> >> +++ b/fs/btrfs/delayed-ref.c
> >> @@ -400,6 +400,20 @@ struct btrfs_delayed_ref_head *btrfs_select_ref_head(
> >> return head;
> >> }
> >>
> >> +void btrfs_delete_ref_head(struct btrfs_delayed_ref_root *delayed_refs,
> >> + struct btrfs_delayed_ref_head *head)
> >> +{
> >> + lockdep_assert_held(&delayed_refs->lock);
> >> + lockdep_assert_held(&head->lock);
> >> +
> >> + rb_erase_cached(&head->href_node, &delayed_refs->href_root);
> >> + RB_CLEAR_NODE(&head->href_node);
> >> + atomic_dec(&delayed_refs->num_entries);
> >> + delayed_refs->num_heads--;
> >> + if (head->processing == 0)
> >> + delayed_refs->num_heads_ready--;
> >
> > num_heads_ready will never execute in cleanup_ref_head, since
> > processing == 0 only when the ref head is unselected. Perhaps those 2
> > lines shouldn't be in this function? I find it a bit confusing that if
> > processing is 0 we decrement num_heads_ready in check_ref_cleanup, but
> > in unselect_delayed_ref_head we set it to 0 and increment it.
> >
> >> +}
> >> +
> >> /*
> >> * Helper to insert the ref_node to the tail or merge with tail.
> >> *
> >> diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
> >> index 8e20c5cb5404..d2af974f68a1 100644
> >> --- a/fs/btrfs/delayed-ref.h
> >> +++ b/fs/btrfs/delayed-ref.h
> >> @@ -261,7 +261,8 @@ static inline void btrfs_delayed_ref_unlock(struct btrfs_delayed_ref_head *head)
> >> {
> >> mutex_unlock(&head->mutex);
> >> }
> >> -
> >> +void btrfs_delete_ref_head(struct btrfs_delayed_ref_root *delayed_refs,
> >> + struct btrfs_delayed_ref_head *head);
> >>
> >> struct btrfs_delayed_ref_head *btrfs_select_ref_head(
> >> struct btrfs_delayed_ref_root *delayed_refs);
> >> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> >> index d242a1174e50..c36b3a42f2bb 100644
> >> --- a/fs/btrfs/extent-tree.c
> >> +++ b/fs/btrfs/extent-tree.c
> >> @@ -2474,12 +2474,9 @@ static int cleanup_ref_head(struct btrfs_trans_handle *trans,
> >> spin_unlock(&delayed_refs->lock);
> >> return 1;
> >> }
> >> - delayed_refs->num_heads--;
> >> - rb_erase_cached(&head->href_node, &delayed_refs->href_root);
> >> - RB_CLEAR_NODE(&head->href_node);
> >> + btrfs_delete_ref_head(delayed_refs, head);
> >> spin_unlock(&head->lock);
> >> spin_unlock(&delayed_refs->lock);
> >> - atomic_dec(&delayed_refs->num_entries);
> >>
> >> trace_run_delayed_ref_head(fs_info, head, 0);
> >>
> >> @@ -6984,22 +6981,9 @@ static noinline int check_ref_cleanup(struct btrfs_trans_handle *trans,
> >> if (!mutex_trylock(&head->mutex))
> >> goto out;
> >>
> >> - /*
> >> - * at this point we have a head with no other entries. Go
> >> - * ahead and process it.
> >> - */
> >> - rb_erase_cached(&head->href_node, &delayed_refs->href_root);
> >> - RB_CLEAR_NODE(&head->href_node);
> >> - atomic_dec(&delayed_refs->num_entries);
> >> -
> >> - /*
> >> - * we don't take a ref on the node because we're removing it from the
> >> - * tree, so we just steal the ref the tree was holding.
> >> - */
> >> - delayed_refs->num_heads--;
> >> - if (head->processing == 0)
> >> - delayed_refs->num_heads_ready--;
> >> + btrfs_delete_ref_head(delayed_refs, head);
> >> head->processing = 0;
>
> On a closer inspection I think here we can do:
>
> ASSERT(head->processing == 0) because at that point we've taken the
> head->lock spinlock which is held during ordinary delayed refs
> processing (in __btrfs_run_delayed_refs) when the head is selected (and
> processing is 1). So head->processing == 0 here I think is a hard
> invariant of the code. The decrement here should pair with the increment
> when the head was initially added to the tree.
>
> In cleanup_ref_head we don't need to ever worry about num_heads_ready
> since it has already been decremented by btrfs_select_ref_head.
>
> As a matter fact this counter is not used anywhere so we might as well
> just remove it.
The logic does not use it, there's only a WARN_ON in
btrfs_select_ref_head, that's more like a debugging or assertion that
everything is fine. So the question is whether to keep it as a
consistency check (and add comments) or remove it and simplify the code.
