On 15.05.2018 20:52, Liu Bo wrote:
> In case of (cow && (p->keep_locks || p->lowest_level)), write_lock_level
> is the max level, and we should grab write lock of root node from the very
> beginning.
THis needs to be expanded to explain what are the adverse effects (if
any) without this commit. And then explain how this commit actually
improve things.
>
> Signed-off-by: Liu Bo <bo....@linux.alibaba.com>
> ---
> fs/btrfs/ctree.c | 29 ++++++++++++++++-------------
> 1 file changed, 16 insertions(+), 13 deletions(-)
>
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index cf34eca41d4e..f7c3f581f647 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -2633,20 +2633,23 @@ static struct extent_buffer
> *btrfs_search_slot_get_root(struct btrfs_root *root,
> goto out;
> }
>
> - /*
> - * we don't know the level of the root node until we actually
> - * have it read locked
> - */
> - b = btrfs_read_lock_root_node(root);
> - level = btrfs_header_level(b);
> - if (level > write_lock_level)
> - goto out;
> + if (write_lock_level < BTRFS_MAX_LEVEL) {
> + /*
> + * we don't know the level of the root node until we actually
> + * have it read locked
> + */
> + b = btrfs_read_lock_root_node(root);
> + level = btrfs_header_level(b);
> + if (level > write_lock_level)
> + goto out;
> +
> + /*
> + * whoops, must trade for write lock
> + */
> + btrfs_tree_read_unlock(b);
> + free_extent_buffer(b);
> + }
Here just seems you are adding 1 extra branch so more context please.
>
> - /*
> - * whoops, must trade for write lock
> - */
> - btrfs_tree_read_unlock(b);
> - free_extent_buffer(b);
> b = btrfs_lock_root_node(root);
> root_lock = BTRFS_WRITE_LOCK;
> /*
>
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