linux-kernel - Re: [PATCH] btrfs: fix return value when race occur between balance and cancel/pause

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Message-ID: <20230810120501.GA2420@suse.cz>
Date:   Thu, 10 Aug 2023 14:05:01 +0200
From:   David Sterba <dsterba@...e.cz>
To:     xiaoshoukui <xiaoshoukui@...il.com>
Cc:     josef@...icpanda.com, dsterba@...e.com, clm@...com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        xiaoshoukui <xiaoshoukui@...jie.com.cn>
Subject: Re: [PATCH] btrfs: fix return value when race occur between balance
 and cancel/pause

On Wed, Aug 09, 2023 at 11:48:10PM -0400, xiaoshoukui wrote:
> Issue a pause or cancel IOCTL request after judging that there is no
> pause or cancel request on the path of __btrfs_balance to return 0,
> which will mislead the user that the pause or cancel requests are
> successful.In fact, the balance request has not been paused or canceled.
> 
> On that race condition, a non-zero errno should be returned to the user.
> 
> Signed-off-by: xiaoshoukui <xiaoshoukui@...jie.com.cn>
> ---
>  fs/btrfs/fs.h      |  6 ++++++
>  fs/btrfs/volumes.c | 14 +++++++++-----
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
> index 203d2a267828..c27def881922 100644
> --- a/fs/btrfs/fs.h
> +++ b/fs/btrfs/fs.h
> @@ -93,6 +93,12 @@ enum {
>  	 */
>  	BTRFS_FS_BALANCE_RUNNING,
>  
> +	/* Indicate that balance has been paused. */
> +	BTRFS_FS_BALANCE_PAUSED,
> +
> +	/* Indicate that balance has been canceled. */
> +	BTRFS_FS_BALANCE_CANCELED,

I don't like that the status is tracked in several bits like that, in
addition to the already complicated locking and state transitions of
restarted balance. I think this is a hint that some things can be
simplified or combined together, though it could be difficult