Re: [PATCH v4] coding-style: Clarify the expectations around bool

[Federico Vaga]

On 2019-01-11 00:48, Jason Gunthorpe wrote:
> There has been some confusion since checkpatch started warning about 
> bool
> use in structures, and people have been avoiding using it.
>
> Many people feel there is still a legitimate place for bool in 
> structures,
> so provide some guidance on bool usage derived from the entire thread 
> that
> spawned the checkpatch warning.
>
> Link:
> https://lkml.kernel.org/r/CA+55aFwVZk1OfB9T2v014PTAKFhtVan_Zj2dOjnCy3x6E4UJfA@xxxxxxxxxxxxxx
> Signed-off-by: Joe Perches <joe@xxxxxxxxxxx>
> Acked-by: Joe Perches <joe@xxxxxxxxxxx>
> Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> ---
>  Documentation/process/coding-style.rst | 38 +++++++++++++++++++++++---
>  scripts/checkpatch.pl                  | 13 ---------
>  2 files changed, 34 insertions(+), 17 deletions(-)
>
> v4:
> - Describe true/false as definitions [Joe]
> - Use clearer language for the _Bool explanation [Bart]
> - Delete the checkpatch tests [Joe]
>
> diff --git a/Documentation/process/coding-style.rst
> b/Documentation/process/coding-style.rst
> index b78dd680c03809..db3e030d0df908 100644
> --- a/Documentation/process/coding-style.rst
> +++ b/Documentation/process/coding-style.rst
> @@ -921,7 +921,37 @@ result.  Typical examples would be functions that
> return pointers; they use
>  NULL or the ERR_PTR mechanism to report failure.
>
>
> -17) Don't re-invent the kernel macros
> +17) Using bool
> +--------------
> +
> +The Linux kernel bool type is an alias for the C99 _Bool type. bool 
> values can
> +only evaluate to 0 or 1, and implicit or explicit conversion to bool
> +automatically converts the value to true or false. When using bool 
> types the
> +!! construction is not needed, which eliminates a class of bugs.
> +
> +When working with bool values the true and false definitions should be 
> used
> +instead of 0 and 1.

A very minor thing. I would suggest to keep consistent, in the 
statement, the mapping
between definitions ("true and false [...]") and their correspondent 
integer values
("[...] instead of 1 and 0").

In few words, I propose to change "0 and 1" into "1 and 0".

> +
> +bool function return types and stack variables are always fine to use 
> whenever
> +appropriate. Use of bool is encouraged to improve readability and is 
> often a
> +better option than 'int' for storing boolean values.
> +
> +Do not use bool if cache line layout or size of the value matters, its 
> size
> +and alignment varies based on the compiled architecture. Structures 
> that are
> +optimized for alignment and size should not use bool.
> +
> +If a structure has many true/false values, consider consolidating them 
> into a
> +bitfield with 1 bit members, or using an appropriate fixed width type, 
> such as
> +u8.
> +
> +Similarly for function arguments, many true/false values can be 
> consolidated
> +into a single bitwise 'flags' argument and 'flags' can often a more 
> readable
> +alternative if the call-sites have naked true/false constants.

Of course, English is not my primary language, but it looks to me that 
here a "be"
is missing: "[...] and 'flags' can often a more readable alternative 
[...]".

> +
> +Otherwise limited use of bool in structures and arguments can improve
> +readability.

I'm going to update the Italian translations for this. Do you want me to 
contribute
directly to this patch? Otherwise I will send a dedicated patch later 
when this one
get accepted.

Thanks

> +18) Don't re-invent the kernel macros
>  -------------------------------------
>
>  The header file include/linux/kernel.h contains a number of macros 
> that
> @@ -944,7 +974,7 @@ need them.  Feel free to peruse that header file
> to see what else is already
>  defined that you shouldn't reproduce in your code.
>
>
> -18) Editor modelines and other cruft
> +19) Editor modelines and other cruft
>  ------------------------------------
>
>  Some editors can interpret configuration information embedded in 
> source files,
> @@ -978,7 +1008,7 @@ own custom mode, or may have some other magic
> method for making indentation
>  work correctly.
>
>
> -19) Inline assembly
> +20) Inline assembly
>  -------------------
>
>  In architecture-specific code, you may need to use inline assembly to 
> interface
> @@ -1010,7 +1040,7 @@ the next instruction in the assembly output:
>  	     : /* outputs */ : /* inputs */ : /* clobbers */);
>
>
> -20) Conditional Compilation
> +21) Conditional Compilation
>  ---------------------------
>
>  Wherever possible, don't use preprocessor conditionals (#if, #ifdef) 
> in .c
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index b737ca9d720441..d62abd032885a1 100755
> --- a/scripts/checkpatch.pl
> +++ b/scripts/checkpatch.pl
> @@ -6368,19 +6368,6 @@ sub process {
>  			}
>  		}
>
> -# check for bool bitfields
> -		if ($sline =~ /^.\s+bool\s*$Ident\s*:\s*\d+\s*;/) {
> -			WARN("BOOL_BITFIELD",
> -			     "Avoid using bool as bitfield.  Prefer bool bitfields as
> unsigned int or u<8|16|32>\n" . $herecurr);
> -		}
> -
> -# check for bool use in .h files
> -		if ($realfile =~ /\.h$/ &&
> -		    $sline =~ /^.\s+bool\s*$Ident\s*(?::\s*d+\s*)?;/) {
> -			CHK("BOOL_MEMBER",
> -			    "Avoid using bool structure members because of possible
> alignment issues - see: https://lkml.org/lkml/2017/11/21/384\n"; .
> $herecurr);
> -		}
> -
>  # check for semaphores initialized locked
>  		if ($line =~ /^.\s*sema_init.+,\W?0\W?\)/) {
>  			WARN("CONSIDER_COMPLETION",

--
Federico Vaga
http://www.federicovaga.it/