On Fri, May 18, 2018 at 11:00:23AM +0800, Liu Bo wrote:
> Typically, when acquiring root node's lock, btrfs tries its best to get
> read lock and trade for write lock if @write_lock_level implies to do so.
>
> In case of (cow && (p->keep_locks || p->lowest_level)), write_lock_level
> is set to BTRFS_MAX_LEVEL, which means we need to acquire root node's
> write lock directly.
>
> In this particular case, the dance of acquiring read lock and then trading
> for write lock can be saved.
>
> Signed-off-by: Liu Bo <bo....@linux.alibaba.com>
Reviewed-by: David Sterba <dste...@suse.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 8d3b09038f37..e619f7e01794 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;
I've added a comment why the check below is done.
> + if (write_lock_level < BTRFS_MAX_LEVEL) {
--
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