linux-kernel - Re: [PATCH] nbd: correct disconnect behavior

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Message-Id: <20130626162107.9237360f7646ec25f23cf5aa@linux-foundation.org>
Date:	Wed, 26 Jun 2013 16:21:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Clements <paul.clements@...eleye.com>
Cc:	<linux-kernel@...r.kernel.org>, <nbd-general@...ts.sourceforge.net>
Subject: Re: [PATCH] nbd: correct disconnect behavior

On Wed, 19 Jun 2013 17:09:18 -0400 (EDT) Paul Clements <paul.clements@...eleye.com> wrote:

> Currently, when a disconnect is requested by the user (via NBD_DISCONNECT
> ioctl) the return from NBD_DO_IT is undefined (it is usually one of
> several error codes). This means that nbd-client does not know if a
> manual disconnect was performed or whether a network error occurred.
> Because of this, nbd-client's persist mode (which tries to reconnect after
> error, but not after manual disconnect) does not always work correctly.
> 
> This change fixes this by causing NBD_DO_IT to always return 0 if a user
> requests a disconnect. This means that nbd-client can correctly either
> persist the connection (if an error occurred) or disconnect (if the user
> requested it).

This sounds like something which users of 3.10 and earlier kernels
might want, so I added the Cc:stable tag.  Please let me know if
you disagree.

> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -623,6 +623,8 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  		if (!nbd->sock)
>  			return -EINVAL;
>  
> +		nbd->disconnect = 1;
> +
>  		nbd_send_req(nbd, &sreq);
>                  return 0;
>  	}
> @@ -654,6 +656,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  				nbd->sock = SOCKET_I(inode);
>  				if (max_part > 0)
>  					bdev->bd_invalidated = 1;
> +				nbd->disconnect = 0; /* we're connected now */
>  				return 0;
>  			} else {
>  				fput(file);
> @@ -742,6 +745,8 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  		set_capacity(nbd->disk, 0);
>  		if (max_part > 0)
>  			ioctl_by_bdev(bdev, BLKRRPART, 0);
> +		if (nbd->disconnect) /* user requested, ignore socket errors */
> +			return 0;
>  		return nbd->harderror;
>  	}

hm, how does nbd work...  Hard to tell as nothing seems to be documented
anywhere :(

afacit the code assumes that the user will run ioctl(NBD_DISCONNECT) and
then ioctl(NBD_DO_IT) and then ioctl(NBD_SET_SOCK), yes?  Does this
change mean that if userspace calls the ioctls in an
other-than-expected order, Weird Things will happen?  Would it be safer
to clear ->disconnect in NBD_DO_IT?

> --- a/include/linux/nbd.h
> +++ b/include/linux/nbd.h
> @@ -41,6 +41,7 @@ struct nbd_device {
>  	u64 bytesize;
>  	pid_t pid; /* pid of nbd-client, if attached */
>  	int xmit_timeout;
> +	int disconnect; /* a disconnect has been requested by user */
>  };

The cool kids are using bool lately ;)
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