Re: [PATCH 4/6] btrfs: add wrapper for conditional start of exclusive operation

[Josef Bacik <josef@toxicpanda.com>]

On 5/21/21 8:06 AM, David Sterba wrote:
> To support optional cancelation of some operations, add helper that will
> wrap all the combinations. In normal mode it's same as
> btrfs_exclop_start, in cancelation mode it checks if it's already
> running and request cancelation and waits until completion.
> 
> The error codes can be returned to to user space and semantics is not
> changed, adding ECANCELED. This should be evaluated as an error and that
> the operation has not completed and the operation should be restarted
> or the filesystem status reviewed.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>   fs/btrfs/ioctl.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 42 insertions(+)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index cacd6ee17d8e..c75ccadf23dc 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -1600,6 +1600,48 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
>   	return ret;
>   }
>   
> +/*
> + * Try to start exclusive operation @type or cancel it if it's running.
> + *
> + * Return:
> + *   0        - normal mode, newly claimed op started
> + *  >0        - normal mode, something else is running,
> + *              return BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS to user space
> + * ECANCELED  - cancel mode, successful cancel
> + * ENOTCONN   - cancel mode, operation not running anymore
> + */
> +static int exclop_start_or_cancel_reloc(struct btrfs_fs_info *fs_info,
> +			enum btrfs_exclusive_operation type, bool cancel)
> +{
> +	if (!cancel) {
> +		/* Start normal op */
> +		if (!btrfs_exclop_start(fs_info, type))
> +			return BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS;
> +		/* Exclusive operation is now claimed */
> +		return 0;
> +	}
> +
> +	/* Cancel running op */
> +	if (btrfs_exclop_start_try_lock(fs_info, type)) {
> +		/*
> +		 * This blocks any exclop finish from setting it to NONE, so we
> +		 * request cancelation. Either it runs and we will wait for it,
> +		 * or it has finished and no waiting will happen.
> +		 */
> +		atomic_inc(&fs_info->reloc_cancel_req);
> +		btrfs_exclop_start_unlock(fs_info);
> +
> +		if (test_bit(BTRFS_FS_RELOC_RUNNING, &fs_info->flags))
> +			wait_on_bit(&fs_info->flags, BTRFS_FS_RELOC_RUNNING,
> +				    TASK_INTERRUPTIBLE);

Do we want to capture the return value here, in case the user hit's ctrl+c we 
can return -EINTR instead so we don't think it succeeded?  Thanks,

Josef