Re: [PATCH v2 1/3] btrfs: scrub: maintain the unlock order in scrub thread

[Filipe Manana <fdmanana@gmail.com>]

On Thu, Nov 29, 2018 at 9:27 AM Anand Jain <anand.jain@oracle.com> wrote:
>
> The device_list_mutex and scrub_lock creates a nested locks in
> btrfs_scrub_dev().
>
> During lock the order is device_list_mutex and then scrub_lock, and during
> unlock, the order is device_list_mutex and then scrub_lock.
> Fix this to the lock order of scrub_lock and then device_list_mutex.
>
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
> v1->v2: change the order of lock acquire first scrub_lock and then
>         device_list_mutex, which matches with the order of unlock.
>         The extra line which are now in the scrub_lock are ok to be
>         under the scrub_lock.

I don't get it.
What problem does this patch fixes?
Doesn't seem any functional fix to me, nor performance gain (by the
contrary, the scrub_lock is now held for a longer time than needed),
nor makes anything more readable or "beautiful".

>  fs/btrfs/scrub.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index 902819d3cf41..a9d6fc3b01d4 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -3813,28 +3813,29 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>                 return -EINVAL;
>         }
>
> -
> +       mutex_lock(&fs_info->scrub_lock);
>         mutex_lock(&fs_info->fs_devices->device_list_mutex);
>         dev = btrfs_find_device(fs_info, devid, NULL, NULL);
>         if (!dev || (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state) &&
>                      !is_dev_replace)) {
>                 mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> +               mutex_unlock(&fs_info->scrub_lock);
>                 return -ENODEV;
>         }
>
>         if (!is_dev_replace && !readonly &&
>             !test_bit(BTRFS_DEV_STATE_WRITEABLE, &dev->dev_state)) {
>                 mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> +               mutex_unlock(&fs_info->scrub_lock);
>                 btrfs_err_in_rcu(fs_info, "scrub: device %s is not writable",
>                                 rcu_str_deref(dev->name));
>                 return -EROFS;
>         }
>
> -       mutex_lock(&fs_info->scrub_lock);
>         if (!test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &dev->dev_state) ||
>             test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &dev->dev_state)) {
> -               mutex_unlock(&fs_info->scrub_lock);
>                 mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> +               mutex_unlock(&fs_info->scrub_lock);
>                 return -EIO;
>         }
>
> @@ -3843,23 +3844,23 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>             (!is_dev_replace &&
>              btrfs_dev_replace_is_ongoing(&fs_info->dev_replace))) {
>                 btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
> -               mutex_unlock(&fs_info->scrub_lock);
>                 mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> +               mutex_unlock(&fs_info->scrub_lock);
>                 return -EINPROGRESS;
>         }
>         btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
>
>         ret = scrub_workers_get(fs_info, is_dev_replace);
>         if (ret) {
> -               mutex_unlock(&fs_info->scrub_lock);
>                 mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> +               mutex_unlock(&fs_info->scrub_lock);
>                 return ret;
>         }
>
>         sctx = scrub_setup_ctx(dev, is_dev_replace);
>         if (IS_ERR(sctx)) {
> -               mutex_unlock(&fs_info->scrub_lock);
>                 mutex_unlock(&fs_info->fs_devices->device_list_mutex);
> +               mutex_unlock(&fs_info->scrub_lock);
>                 scrub_workers_put(fs_info);
>                 return PTR_ERR(sctx);
>         }
> --
> 1.8.3.1



-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”