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/[email protected]
Signed-off-by: Jim Cromie <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Dwaipayan Ray <[email protected]>
Cc: Lukas Bulwahn <[email protected]>
Cc: Louis Chauvet <[email protected]>
Cc: Viresh Kumar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jimc

scripts/checkpatch.pl

@@ -151,6 +151,24 @@ sub help {
 	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");
 				}
 
 			}