linux-kernel - [PATCH 3/4] Documentation/process: Add fallthrough pseudo-keyword

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Message-Id: <09a42c7275afa7e6e9e3fc57a15122201fccd6f7.1570292505.git.joe@perches.com>
Date:   Sat,  5 Oct 2019 09:46:43 -0700
From:   Joe Perches <joe@...ches.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Pavel Machek <pavel@....cz>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Shawn Landden <shawn@....icu>, x86@...nel.org,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        clang-built-linux@...glegroups.com,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: [PATCH 3/4] Documentation/process: Add fallthrough pseudo-keyword

Describe the fallthrough pseudo-keyword.

Convert the coding-style.rst example to the keyword style.
Add description and links to deprecated.rst.

Signed-off-by: Joe Perches <joe@...ches.com>
---
 Documentation/process/coding-style.rst |  2 +-
 Documentation/process/deprecated.rst   | 33 +++++++++++++++++++++++----------
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index f4a2198187f9..ada573b7d703 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -56,7 +56,7 @@ instead of ``double-indenting`` the ``case`` labels.  E.g.:
 	case 'K':
 	case 'k':
 		mem <<= 10;
-		/* fall through */
+		fallthrough;
 	default:
 		break;
 	}
diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index 56280e108d5a..a0ffdc8daef3 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -122,14 +122,27 @@ memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
 
 Implicit switch case fall-through
 ---------------------------------
-The C language allows switch cases to "fall through" when
-a "break" statement is missing at the end of a case. This,
-however, introduces ambiguity in the code, as it's not always
-clear if the missing break is intentional or a bug. As there
-have been a long list of flaws `due to missing "break" statements
+The C language allows switch cases to "fall-through" when a "break" statement
+is missing at the end of a case. This, however, introduces ambiguity in the
+code, as it's not always clear if the missing break is intentional or a bug.
+
+As there have been a long list of flaws `due to missing "break" statements
 <https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
-"implicit fall-through". In order to identify an intentional fall-through
-case, we have adopted the marking used by static analyzers: a comment
-saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
-is more widely handled by C compilers, static analyzers, and IDEs, we can
-switch to using that instead.
+"implicit fall-through".
+
+In order to identify intentional fall-through cases, we have adopted a
+pseudo-keyword macro 'fallthrough' which expands to gcc's extension
+__attribute__((__fallthrough__)).  `Statement Attributes
+<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_
+
+When the C17/C18  [[fallthrough]] syntax is more commonly supported by
+C compilers, static analyzers, and IDEs, we can switch to using that syntax
+for the macro pseudo-keyword.
+
+All switch/case blocks must end in one of:
+
+	break;
+	fallthrough;
+	continue;
+	goto <label>;
+	return [expression];
-- 
2.15.0