Re: New sparse warnings from sched.h

From: luca abeni
Date: Wed Nov 15 2017 - 03:36:48 EST


Hi,

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

> commit 799ba82de01e7543f6b2042e1a739f3a20255f23
> Author: luca abeni <luca.abeni@xxxxxxxxxxxxxxx>
> 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