linux-kernel - Re: [PATCH -mm] DC395x SCSI driver: Shut up uninitialized variable build warning

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Message-ID: <46DB1BDF.4090100@garzik.org>
Date:	Sun, 02 Sep 2007 16:23:59 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Satyam Sharma <satyam@...radead.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	James Bottomley <james.bottomley@...eleye.com>,
	Oliver Neukum <oliver@...kum.name>, linux-scsi@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH -mm] DC395x SCSI driver: Shut up uninitialized variable
 build warning

Satyam Sharma wrote:
> drivers/scsi/dc395x.c: In function ‘dc395x_init_one’:
> drivers/scsi/dc395x.c:4272: warning: ‘ptr’ may be used uninitialized in this function
> 
> has been verified to be a bogus warning. Let's shut it up.
> 
> Signed-off-by: Satyam Sharma <satyam@...radead.org>
> 
> ---
> 
>  drivers/scsi/dc395x.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- linux-2.6.23-rc4-mm1/drivers/scsi/dc395x.c~fix	2007-09-02 20:57:51.000000000 +0530
> +++ linux-2.6.23-rc4-mm1/drivers/scsi/dc395x.c	2007-09-02 21:10:49.000000000 +0530
> @@ -4269,7 +4269,7 @@ static int __devinit adapter_sg_tables_a
>  	const unsigned srbs_per_page = PAGE_SIZE/SEGMENTX_LEN;
>  	int srb_idx = 0;
>  	unsigned i = 0;
> -	struct SGentry *ptr;
> +	struct SGentry * uninitialized_var(ptr);

For things like this, we need to see (perhaps a separate email in the 
same thread) your build details:  config, arch, compiler version, etc.

This warning does not appear on x86 or x86-64 here, with the current 
version of gcc.  And I'm not so sure we want to be adding these markers 
for older versions of the compiler, or for what is ultimately a 
temporary [situation | configuration] in the grand scheme of things.

I paid careful attention to my recent set of uninitialized_var() markers 
(now upstream), making sure that they persisted across several compiler 
versions, ensuring the warning was not a temporary optimizer bug quickly 
remedied.

I hope to encourage similar diligence in others :)  These markers have 
the very-real potential downside of hiding bugs, so they should be used 
sparingly.

	Jeff

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