linux-kernel - Re: [PATCH] xen/blkback: prevent repeated backend

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Message-Id: <50C9FC0302000078000B0344@nat28.tlf.novell.com>
Date:	Thu, 13 Dec 2012 15:02:11 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Olaf Hering" <olaf@...fle.de>, <konrad.wilk@...cle.com>
Cc:	<xen-devel@...ts.xen.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen/blkback: prevent repeated backend_changed
 invocations

>>> On 11.12.12 at 21:50, Olaf Hering <olaf@...fle.de> wrote:
> backend_changed might be called multiple times, which will leak
> be->mode. Make sure it will be called only once. Remove some unneeded
> checks. Also the be->mode string was leaked, release the memory on
> device shutdown.

So I decided to make an attempt myself, retaining the current
behavior of allowing multiple calls, yet not having to sprinkle
around multiple kfree()-s for be->mode. Slightly re-structuring
the function made this pretty strait forward.

Jan

> Signed-off-by: Olaf Hering <olaf@...fle.de>
> ---
> 
> incorporate all comments from Jan.
> fold the oneline change to xen_blkbk_remove into this change
> now its compile tested.
> 
>  drivers/block/xen-blkback/xenbus.c | 69 ++++++++++++++++++--------------------
>  1 file changed, 33 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/xenbus.c 
> b/drivers/block/xen-blkback/xenbus.c
> index f58434c..5ca77c3 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -28,6 +28,7 @@ struct backend_info {
>  	unsigned		major;
>  	unsigned		minor;
>  	char			*mode;
> +	unsigned		alive;
>  };
>  
>  static struct kmem_cache *xen_blkif_cachep;
> @@ -366,6 +367,7 @@ static int xen_blkbk_remove(struct xenbus_device *dev)
>  		be->blkif = NULL;
>  	}
>  
> +	kfree(be->mode);
>  	kfree(be);
>  	dev_set_drvdata(&dev->dev, NULL);
>  	return 0;
> @@ -501,10 +503,14 @@ static void backend_changed(struct xenbus_watch *watch,
>  		= container_of(watch, struct backend_info, backend_watch);
>  	struct xenbus_device *dev = be->dev;
>  	int cdrom = 0;
> -	char *device_type;
> +	char *device_type, *p;
> +	long handle;
>  
>  	DPRINTK("");
>  
> +	if (be->alive)
> +		return;
> +
>  	err = xenbus_scanf(XBT_NIL, dev->nodename, "physical-device", "%x:%x",
>  			   &major, &minor);
>  	if (XENBUS_EXIST_ERR(err)) {
> @@ -520,12 +526,7 @@ static void backend_changed(struct xenbus_watch *watch,
>  		return;
>  	}
>  
> -	if ((be->major || be->minor) &&
> -	    ((be->major != major) || (be->minor != minor))) {
> -		pr_warn(DRV_PFX "changing physical device (from %x:%x to %x:%x) not 
> supported.\n",
> -			be->major, be->minor, major, minor);
> -		return;
> -	}
> +	be->alive = 1;
>  
>  	be->mode = xenbus_read(XBT_NIL, dev->nodename, "mode", NULL);
>  	if (IS_ERR(be->mode)) {
> @@ -541,39 +542,35 @@ static void backend_changed(struct xenbus_watch *watch,
>  		kfree(device_type);
>  	}
>  
> -	if (be->major == 0 && be->minor == 0) {
> -		/* Front end dir is a number, which is used as the handle. */
> -
> -		char *p = strrchr(dev->otherend, '/') + 1;
> -		long handle;
> -		err = strict_strtoul(p, 0, &handle);
> -		if (err)
> -			return;
> -
> -		be->major = major;
> -		be->minor = minor;
> +	/* Front end dir is a number, which is used as the handle. */
> +	p = strrchr(dev->otherend, '/') + 1;
> +	err = strict_strtoul(p, 0, &handle);
> +	if (err)
> +		return;
>  
> -		err = xen_vbd_create(be->blkif, handle, major, minor,
> -				 (NULL == strchr(be->mode, 'w')), cdrom);
> -		if (err) {
> -			be->major = 0;
> -			be->minor = 0;
> -			xenbus_dev_fatal(dev, err, "creating vbd structure");
> -			return;
> -		}
> +	be->major = major;
> +	be->minor = minor;
>  
> -		err = xenvbd_sysfs_addif(dev);
> -		if (err) {
> -			xen_vbd_free(&be->blkif->vbd);
> -			be->major = 0;
> -			be->minor = 0;
> -			xenbus_dev_fatal(dev, err, "creating sysfs entries");
> -			return;
> -		}
> +	err = xen_vbd_create(be->blkif, handle, major, minor,
> +			 (NULL == strchr(be->mode, 'w')), cdrom);
> +	if (err) {
> +		be->major = 0;
> +		be->minor = 0;
> +		xenbus_dev_fatal(dev, err, "creating vbd structure");
> +		return;
> +	}
>  
> -		/* We're potentially connected now */
> -		xen_update_blkif_status(be->blkif);
> +	err = xenvbd_sysfs_addif(dev);
> +	if (err) {
> +		xen_vbd_free(&be->blkif->vbd);
> +		be->major = 0;
> +		be->minor = 0;
> +		xenbus_dev_fatal(dev, err, "creating sysfs entries");
> +		return;
>  	}
> +
> +	/* We're potentially connected now */
> +	xen_update_blkif_status(be->blkif);
>  }
>  
>  
> -- 
> 1.8.0.1


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