RE: [PATCH] omap2/omapfb: make DBG() more resistant in if-elseconstructions

[Premi, Sanjeev]

> -----Original Message-----
> From: linux-omap-owner@xxxxxxxxxxxxxxx 
> [mailto:linux-omap-owner@xxxxxxxxxxxxxxx] On Behalf Of Niels de Vos
> Sent: Tuesday, May 10, 2011 2:51 PM
> To: linux-omap@xxxxxxxxxxxxxxx
> Cc: linux-fbdev@xxxxxxxxxxxxxxx; 
> linux-kernel@xxxxxxxxxxxxxxx; Niels de Vos
> Subject: [PATCH] omap2/omapfb: make DBG() more resistant in 
> if-else constructions
> 
> When DBG() is used in a simple if-else, the resulting code path
> currently depends on the definition of DBG(). Inserting the 
> statement in
> a "do { ... } while (0)" prevents this possible misuse.
> 
> Signed-off-by: Niels de Vos <ndevos@xxxxxxxxxx>
> 
> ---
> Note, I have not found any offenders, but a mistake can 
> easily be made.
> The following example shows what can go wrong if little intention is
> paid to the definition of the DBG() macro.
> 
> Example:
> 	if something_went_wrong()
> 		DBG("oh no, something went wrong!\n");
> 	else
> 		printk("all went fine\n");
> 
> Old result where the else is placed inside the first if-statment:
> 	if something_went_wrong() {
> 		if (omapfb_debug) {
> 			printk(KERN_DEBUG "oh no, something 
> went wrong!\n");
> 		} else {
> 			printk("all went fine\n");
> 		}
> 	}
> 
> New result where the else is an alternative to the first if-statement:
> 	if something_went_wrong() {
> 		do {
> 			if (omapfb_debug)
> 				printk(KERN_DEBUG "oh no, 
> something went wrong!\n");
> 		} while (0);
> 	} else {
> 		printk("all went fine\n");
> 	}
> ---
>  drivers/video/omap2/omapfb/omapfb.h |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/video/omap2/omapfb/omapfb.h 
> b/drivers/video/omap2/omapfb/omapfb.h
> index 1305fc9..a01b0bb 100644
> --- a/drivers/video/omap2/omapfb/omapfb.h
> +++ b/drivers/video/omap2/omapfb/omapfb.h
> @@ -34,8 +34,10 @@
>  #ifdef DEBUG
>  extern unsigned int omapfb_debug;
>  #define DBG(format, ...) \
> -	if (omapfb_debug) \
> -		printk(KERN_DEBUG "OMAPFB: " format, ## __VA_ARGS__)
> +	do { \
> +		if (omapfb_debug) \
> +			printk(KERN_DEBUG "OMAPFB: " format, ## 
> __VA_ARGS__); \
> +	while (0)

A real good find. Wondering if it really didn't create any problems so far...

~sanjeev

>  #else
>  #define DBG(format, ...)
>  #endif
> -- 
> 1.7.4.4
> 
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