linux-kernel - Re: New sparse warnings from sched.h

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Message-ID: <20171115093629.79f8ab74@sweethome>
Date:   Wed, 15 Nov 2017 09:36:29 +0100
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: New sparse warnings from sched.h

Hi,

On Tue, 14 Nov 2017 12:41:35 -0800
Matthew Wilcox <willy@...radead.org> wrote:

> commit 799ba82de01e7543f6b2042e1a739f3a20255f23
> Author: luca abeni <luca.abeni@...tannapisa.it>
> Date:   Thu Sep 7 12:09:31 2017 +0200
> 
>     sched/deadline: Use C bitfields for the state flags
>     
>     Ask the compiler to use a single bit for storing true / false
> values, instead of wasting the size of a whole int value.
>     Tested with gcc 5.4.0 on x86_64, and the compiler produces the
> expected Assembly (similar to the Assembly code generated when
> explicitly accessing the bits with bitmasks, "&" and "|").
> 
> produces four warnings from sparse for every file which includes
> sched.h:
> 
> ./include/linux/sched.h:476:62: error: dubious one-bit signed bitfield
> ./include/linux/sched.h:477:62: error: dubious one-bit signed bitfield
> ./include/linux/sched.h:478:62: error: dubious one-bit signed bitfield
> ./include/linux/sched.h:479:62: error: dubious one-bit signed bitfield
> 
> This seems like the trivial fix (untested):
[...]

I think Dan Carpenter sent a similar patch (and I replied agreeing with
it).

I believe in this particular case signed integers are safe, but if
"unsigned int" is the preferred style in the kernel I agree with the
change.


			Thanks,
				Luca

> 
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index a5dc7c98b0a2..21991d668d35 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -473,10 +473,10 @@ struct sched_dl_entity {
>  	 * conditions between the inactive timer handler and the
> wakeup
>  	 * code.
>  	 */
> -	int				dl_throttled      : 1;
> -	int				dl_boosted        : 1;
> -	int				dl_yielded        : 1;
> -	int				dl_non_contending : 1;
> +	unsigned int			dl_throttled      : 1;
> +	unsigned int			dl_boosted        : 1;
> +	unsigned int			dl_yielded        : 1;
> +	unsigned int			dl_non_contending : 1;
>  
>  	/*
>  	 * Bandwidth enforcement timer. Each -deadline task has its