[PATCH v3 0/2] codingstyle: avoid unused parameters for a function-like macro

[PATCH v3 0/2] codingstyle: avoid unused parameters for a function-like macro

[Barry Song]

From: Barry Song <v-songbaohua@oppo.com>


A function-like macro could result in build warnings such as
"unused variable." This patchset updates the guidance to
recommend always using a static inline function instead
and also provides checkpatch support for this new rule.

Barry Song (1):
  Documentation: coding-style: ask function-like macros to evaluate
    parameters

Xining Xu (1):
  scripts: checkpatch: Check unused parameters for function-like macro

 Documentation/process/coding-style.rst | 16 ++++++++++++++++
 scripts/checkpatch.pl                  | 24 ++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

-- 
2.34.1

[PATCH v3 1/2] Documentation: coding-style: ask function-like macros to evaluate parameters

[Barry Song]

From: Barry Song <v-songbaohua@oppo.com>

Recent commit 77292bb8ca69c80 ("crypto: scomp - remove memcpy if
sg_nents is 1 and pages are lowmem") leads to warnings on xtensa
and loongarch,
   In file included from crypto/scompress.c:12:
   include/crypto/scatterwalk.h: In function 'scatterwalk_pagedone':
   include/crypto/scatterwalk.h:76:30: warning: variable 'page' set but not used [-Wunused-but-set-variable]
      76 |                 struct page *page;
         |                              ^~~~
   crypto/scompress.c: In function 'scomp_acomp_comp_decomp':
>> crypto/scompress.c:174:38: warning: unused variable 'dst_page' [-Wunused-variable]
     174 |                         struct page *dst_page = sg_page(req->dst);
         |

The reason is that flush_dcache_page() is implemented as a noop
macro on these platforms as below,

 #define flush_dcache_page(page) do { } while (0)

The driver code, for itself, seems be quite innocent and placing
maybe_unused seems pointless,

 struct page *dst_page = sg_page(req->dst);

 for (i = 0; i < nr_pages; i++)
 	flush_dcache_page(dst_page + i);

And it should be independent of architectural implementation
differences.

Let's provide guidance on coding style for requesting parameter
evaluation or proposing the migration to a static inline
function.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Mark Brown <broonie@kernel.org>
Suggested-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 Documentation/process/coding-style.rst | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 9c7cf7347394..791d333a57fd 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -827,6 +827,22 @@ Macros with multiple statements should be enclosed in a do - while block:
 				do_this(b, c);		\
 		} while (0)
 
+Function-like macros with unused parameters should be replaced by static
+inline functions to avoid the issue of unused variables:
+
+.. code-block:: c
+
+	static inline void fun(struct foo *foo)
+	{
+	}
+
+For historical reasons, many files still use the cast to (void) to evaluate
+parameters, but this method is not recommended:
+
+.. code-block:: c
+
+	#define macrofun(foo) do { (void) (foo); } while (0)
+
 Things to avoid when using macros:
 
 1) macros that affect control flow:
-- 
2.34.1

Re: [PATCH v3 1/2] Documentation: coding-style: ask function-like macros to evaluate parameters

[Mark Brown]

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On Fri, Mar 22, 2024 at 09:49:36PM +1300, Barry Song wrote:

> The driver code, for itself, seems be quite innocent and placing
> maybe_unused seems pointless,
> 
>  struct page *dst_page = sg_page(req->dst);
> 
>  for (i = 0; i < nr_pages; i++)
>  	flush_dcache_page(dst_page + i);
> 
> And it should be independent of architectural implementation
> differences.
> 
> Let's provide guidance on coding style for requesting parameter
> evaluation or proposing the migration to a static inline
> function.

Reviewed-by: Mark Brown <broonie@kernel.org>

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[PATCH v3 2/2] scripts: checkpatch: Check unused parameters for function-like macro

[Barry Song]

From: Xining Xu <ma.xxn@outlook.com>

If function-like macros do not utilize a parameter, it might result in a
build warning. In our coding style guidelines, we advocate for utilizing
static inline functions to replace such macros. This patch verifies
compliance with the new rule.

For a macro such as the one below,

 #define test(a) do { } while (0)

The test result is as follows.

 ERROR: Parameter 'a' is not used in function-like macro, please use static
 inline instead
 #21: FILE: mm/init-mm.c:20:
 +#define test(a) do { } while (0)

 total: 1 errors, 0 warnings, 8 lines checked

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Xining Xu <ma.xxn@outlook.com>
Tested-by: Barry Song <v-songbaohua@oppo.com>
---
 scripts/checkpatch.pl | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 9c4c4a61bc83..6f778f3403b5 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -6109,6 +6109,30 @@ sub process {
 				WARN("TRAILING_SEMICOLON",
 				     "macros should not use a trailing semicolon\n" . "$herectx");
 			}
+
+			if ($dstat =~ /^\+\s*#\s*define\s+$Ident\s*(\((?:[^\(\)]++|(?-1))*\))\s+(\S+.*)(\/\/.*)?/) {
+				my $params = $1 || "";
+				my $body = $2 || "";
+
+			    # get the individual params
+				$params =~ tr/()//d;
+				# remove leading and trailing whitespace
+				$params =~ s/^\s+|\s+$//g;
+
+				$ctx =~ s/\n*$//;
+				my $cnt = statement_rawlines($ctx);
+				my $herectx = get_stat_here($linenr, $cnt, $here);
+
+				if ($params ne "") {
+					my @paramList = split /,\s*/, $params;
+					foreach my $param(@paramList) {
+						if ($body !~ /\b$param\b/) {
+							ERROR("UNUSED_PARAM_IN_MACRO",
+							     "Parameter '$param' is not used in function-like macro, please use static inline instead\n" . "$herectx");
+						}
+					}
+				}
+			}
 		}
 
 # check for redundant bracing round if etc
-- 
2.34.1

Re: [PATCH v3 2/2] scripts: checkpatch: check unused parameters for function-like macro

[Charlemagne Lasse]

Hi,

Can this patch please be dropped from Linux-next (currently via
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm#mm-everything)?
It is obviously wrong when dealing with named variadic macro
parameters:

 ./scripts/checkpatch.pl -f include/linux/rculist.h
ERROR: Parameter 'dummy' is not used in function-like macro, please
use static inline instead
#51: FILE: include/linux/rculist.h:51:
+#define __list_check_rcu(dummy, cond, extra...)
         \
+       ({                                                              \
+       check_arg_count_one(extra);                                     \
+       RCU_LOCKDEP_WARN(!(cond) && !rcu_read_lock_any_held(),          \
+                        "RCU-list traversed in non-reader section!");  \
+       })

ERROR: Parameter 'extra...' is not used in function-like macro, please
use static inline instead
#51: FILE: include/linux/rculist.h:51:
+#define __list_check_rcu(dummy, cond, extra...)
         \
+       ({                                                              \
+       check_arg_count_one(extra);                                     \
+       RCU_LOCKDEP_WARN(!(cond) && !rcu_read_lock_any_held(),          \
+                        "RCU-list traversed in non-reader section!");  \
+       })

ERROR: Parameter 'dummy' is not used in function-like macro, please
use static inline instead
#64: FILE: include/linux/rculist.h:64:
+#define __list_check_rcu(dummy, cond, extra...)
         \
+       ({ check_arg_count_one(extra); })

ERROR: Parameter 'cond' is not used in function-like macro, please use
static inline instead
#64: FILE: include/linux/rculist.h:64:
+#define __list_check_rcu(dummy, cond, extra...)
         \
+       ({ check_arg_count_one(extra); })

ERROR: Parameter 'extra...' is not used in function-like macro, please
use static inline instead
#64: FILE: include/linux/rculist.h:64:
+#define __list_check_rcu(dummy, cond, extra...)
         \
+       ({ check_arg_count_one(extra); })

Re: [PATCH v3 2/2] scripts: checkpatch: check unused parameters for function-like macro

[Joe Perches]

On Sat, 2024-03-30 at 00:08 +0100, Charlemagne Lasse wrote:
> Hi,
> 
> Can this patch please be dropped from Linux-next (currently via
> git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm#mm-everything)?
> It is obviously wrong when dealing with named variadic macro
> parameters:

I agree it was a bit early to add to -mm but
there is a fix for (most of) that coming up.