linux-kernel - Re: [PATCH 2/4] virtio_blk: Less function calls in init

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Date:   Tue, 13 Sep 2016 14:54:58 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     SF Markus Elfring <elfring@...rs.sourceforge.net>,
        virtualization@...ts.linux-foundation.org,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Julia Lawall <julia.lawall@...6.fr>,
        kernel-janitors@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] virtio_blk: Less function calls in init_vq() after
 error detection

On 09/13/2016 02:13 PM, SF Markus Elfring wrote:
> From: Markus Elfring <elfring@...rs.sourceforge.net>
> Date: Tue, 13 Sep 2016 13:20:44 +0200
> 
> The kfree() function was called in up to three cases
> by the init_vq() function during error handling even if
> the passed variable contained a null pointer.
> 
> * Split a condition check for memory allocation failures.
> 
> * Adjust jump targets according to the Linux coding style convention.
> 
> Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> ---
>  drivers/block/virtio_blk.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)

Can't you see from this diffstat that the patch actually seems to makes
the code more complex?
In addition, please have a look at commit 347a529398e8e723338cca5d8a8ae2d9e7e93448
    virtio_blk: Fix a slient kernel panic

which did the opposite of your patch. And in fact it fixed a bug. Quite obviously
multiple labels are harder to read and harder to get right. For error handling with
just kfree one label is just the right thing to.

> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 6553eb7..d28dbcf 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -395,11 +395,21 @@ static int init_vq(struct virtio_blk *vblk)
>  		return -ENOMEM;
> 
>  	names = kmalloc_array(num_vqs, sizeof(*names), GFP_KERNEL);
> +	if (!names) {
> +		err = -ENOMEM;
> +		goto free_vblk_vqs;
> +	}
> +
>  	callbacks = kmalloc_array(num_vqs, sizeof(*callbacks), GFP_KERNEL);
> +	if (!callbacks) {
> +		err = -ENOMEM;
> +		goto free_names;
> +	}
> +
>  	vqs = kmalloc_array(num_vqs, sizeof(*vqs), GFP_KERNEL);
> -	if (!names || !callbacks || !vqs) {
> +	if (!vqs) {
>  		err = -ENOMEM;
> -		goto out;
> +		goto free_callbacks;
>  	}
> 
>  	for (i = 0; i < num_vqs; i++) {
> @@ -411,19 +421,21 @@ static int init_vq(struct virtio_blk *vblk)
>  	/* Discover virtqueues and write information to configuration.  */
>  	err = vdev->config->find_vqs(vdev, num_vqs, vqs, callbacks, names);
>  	if (err)
> -		goto out;
> +		goto free_vqs;
> 
>  	for (i = 0; i < num_vqs; i++) {
>  		spin_lock_init(&vblk->vqs[i].lock);
>  		vblk->vqs[i].vq = vqs[i];
>  	}
>  	vblk->num_vqs = num_vqs;
> -
> -out:
> + free_vqs:
>  	kfree(vqs);
> + free_callbacks:
>  	kfree(callbacks);
> + free_names:
>  	kfree(names);
>  	if (err)
> + free_vblk_vqs:
>  		kfree(vblk->vqs);
>  	return err;
>  }
>