linux-kernel - Re: [PATCH 08/20] media: pci: saa7164.c: Replace if(cond) BUG with BUG

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Date:   Sat, 29 Aug 2020 08:44:14 +0200
From:   Mauro Carvalho Chehab <mchehab@...nel.org>
To:     "Daniel W. S. Almeida" <dwlsalmeida@...il.com>
Cc:     linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
        linux-media@...r.kernel.org
Subject: Re: [PATCH 08/20] media: pci: saa7164.c: Replace if(cond) BUG with
 BUG_ON

Hi Daniel,

Em Fri,  7 Aug 2020 05:35:35 -0300
"Daniel W. S. Almeida" <dwlsalmeida@...il.com> escreveu:

> From: "Daniel W. S. Almeida" <dwlsalmeida@...il.com>
> 
> Fix the following coccinelle reports:
> 
> drivers/media/pci/saa7164/saa7164-buffer.c:254:3-6: WARNING: Use BUG_ON
> instead of if condition followed by BUG.
> 
> drivers/media/pci/saa7164/saa7164-buffer.c:261:3-6: WARNING: Use BUG_ON
> instead of if condition followed by BUG.
> 
> Found using - Coccinelle (http://coccinelle.lip6.fr)

I ended accepting those patches, but IMO, this doesn't make
the code any better.

The thing is that very few parts of the Kernel should panic if
things go sideways. I don't think that any media driver would
apply on such cases.

As pointed at:

	https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

The real fix is to use WARN_ON(), probably with something like:

	if (WARN_ON(something))
		return;

as probably the code after BUG() would be assuming that the
condition was asserted.

Regards,
Mauro

> 
> Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@...il.com>
> ---
>  drivers/media/pci/saa7164/saa7164-buffer.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/media/pci/saa7164/saa7164-buffer.c b/drivers/media/pci/saa7164/saa7164-buffer.c
> index 289cb901985b..245d9db280aa 100644
> --- a/drivers/media/pci/saa7164/saa7164-buffer.c
> +++ b/drivers/media/pci/saa7164/saa7164-buffer.c
> @@ -250,15 +250,14 @@ int saa7164_buffer_cfg_port(struct saa7164_port *port)
>  	list_for_each_safe(c, n, &port->dmaqueue.list) {
>  		buf = list_entry(c, struct saa7164_buffer, list);
>  
> -		if (buf->flags != SAA7164_BUFFER_FREE)
> -			BUG();
> +		BUG_ON(buf->flags != SAA7164_BUFFER_FREE);
>  
>  		/* Place the buffer in the h/w queue */
>  		saa7164_buffer_activate(buf, i);
>  
>  		/* Don't exceed the device maximum # bufs */
> -		if (i++ > port->hwcfg.buffercount)
> -			BUG();
> +		BUG_ON(i > port->hwcfg.buffercount);
> +		i++;
>  
>  	}
>  	mutex_unlock(&port->dmaqueue_lock);
> @@ -302,4 +301,3 @@ void saa7164_buffer_dealloc_user(struct saa7164_user_buffer *buf)
>  
>  	kfree(buf);
>  }
> -



Thanks,
Mauro