checkpatch: qualify do-while-0 advice

Add a paragraph of advice qualifying the general do-while-0 advice, noting
3 possible misguidings.  reduce one ERROR to WARN, for the case I actually
encountered.

And add 'static_assert' to named exceptions, along with some additional
comments about named exceptions vs (detection of) declarative construction
primitives (union, struct, [], etc).

Link: https://lkml.kernel.org/r/20250325235156.663269-3-jim.cromie@gmail.com
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Louis Chauvet <louis.chauvet@bootlin.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 75349f766b89d19b519a90be8659517f5ee960dc..e8afe3f765de52e80cd3b51fe1d6d50cb9da3dbf 100755 (executable)
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -151,6 +151,24 @@ EOM
        exit($exitcode);
 }
 
+my $DO_WHILE_0_ADVICE = q{
+   do {} while (0) advice is over-stated in a few situations:
+
+   The more obvious case is macros, like MODULE_PARM_DESC, invoked at
+   file-scope, where C disallows code (it must be in functions).  See
+   $exceptions if you have one to add by name.
+
+   More troublesome is declarative macros used at top of new scope,
+   like DECLARE_PER_CPU.  These might just compile with a do-while-0
+   wrapper, but would be incorrect.  Most of these are handled by
+   detecting struct,union,etc declaration primitives in $exceptions.
+
+   Theres also macros called inside an if (block), which "return" an
+   expression.  These cannot do-while, and need a ({}) wrapper.
+
+   Enjoy this qualification while we work to improve our heuristics.
+};
+
 sub uniq {
        my %seen;
        return grep { !$seen{$_}++ } @_;
@@ -5883,9 +5901,9 @@ sub process {
                        }
                }
 
-# multi-statement macros should be enclosed in a do while loop, grab the
-# first statement and ensure its the whole macro if its not enclosed
-# in a known good container
+# Usually multi-statement macros should be enclosed in a do {} while
+# (0) loop.  Grab the first statement and ensure its the whole macro
+# if its not enclosed in a known good container
                if ($realfile !~ m@/vmlinux.lds.h$@ &&
                    $line =~ /^.\s*\#\s*define\s*$Ident(\()?/) {
                        my $ln = $linenr;
@@ -5938,10 +5956,13 @@ sub process {
 
                        my $exceptions = qr{
                                $Declare|
+                               # named exceptions
                                module_param_named|
                                MODULE_PARM_DESC|
                                DECLARE_PER_CPU|
                                DEFINE_PER_CPU|
+                               static_assert|
+                               # declaration primitives
                                __typeof__\(|
                                union|
                                struct|
@@ -5976,11 +5997,11 @@ sub process {
                                        ERROR("MULTISTATEMENT_MACRO_USE_DO_WHILE",
                                              "Macros starting with if should be enclosed by a do - while loop to avoid possible if/else logic defects\n" . "$herectx");
                                } elsif ($dstat =~ /;/) {
-                                       ERROR("MULTISTATEMENT_MACRO_USE_DO_WHILE",
-                                             "Macros with multiple statements should be enclosed in a do - while loop\n" . "$herectx");
+                                       WARN("MULTISTATEMENT_MACRO_USE_DO_WHILE",
+                                             "Non-declarative macros with multiple statements should be enclosed in a do - while loop\n" . "$herectx\nBUT SEE:\n$DO_WHILE_0_ADVICE");
                                } else {
                                        ERROR("COMPLEX_MACRO",
-                                             "Macros with complex values should be enclosed in parentheses\n" . "$herectx");
+                                             "Macros with complex values should be enclosed in parentheses\n" . "$herectx\nBUT SEE:\n$DO_WHILE_0_ADVICE");
                                }
 
                        }