1 files changed, 65 insertions, 5 deletions

diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index 3171822c22a5..618a33c940df 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -392,7 +392,12 @@ The goto statement comes in handy when a function exits from multiple
 locations and some common work such as cleanup has to be done.  If there is no
 cleanup needed then just return directly.
 
-The rationale is:
+Choose label names which say what the goto does or why the goto exists.  An
+example of a good name could be "out_buffer:" if the goto frees "buffer".  Avoid
+using GW-BASIC names like "err1:" and "err2:".  Also don't name them after the
+goto location like "err_kmalloc_failed:"
+
+The rationale for using gotos is:
 
 - unconditional statements are easier to understand and follow
 - nesting is reduced
@@ -403,9 +408,10 @@ The rationale is:
 int fun(int a)
 {
         int result = 0;
-        char *buffer = kmalloc(SIZE);
+        char *buffer;
 
-        if (buffer == NULL)
+        buffer = kmalloc(SIZE, GFP_KERNEL);
+        if (!buffer)
                 return -ENOMEM;
 
         if (condition1) {
@@ -413,14 +419,25 @@ int fun(int a)
                         ...
                 }
                 result = 1;
-                goto out;
+                goto out_buffer;
         }
         ...
-out:
+out_buffer:
         kfree(buffer);
         return result;
 }
 
+A common type of bug to be aware of it "one err bugs" which look like this:
+
+err:
+        kfree(foo->bar);
+        kfree(foo);
+        return ret;
+
+The bug in this code is that on some exit paths "foo" is NULL.  Normally the
+fix for this is to split it up into two error labels "err_bar:" and "err_foo:".
+
+
                 Chapter 8: Commenting
 
 Comments are good, but there is also a danger of over-commenting.  NEVER
@@ -845,6 +862,49 @@ next instruction in the assembly output:
              : /* outputs */ : /* inputs */ : /* clobbers */);
 
 
+                Chapter 20: Conditional Compilation
+
+Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
+files; doing so makes code harder to read and logic harder to follow.  Instead,
+use such conditionals in a header file defining functions for use in those .c
+files, providing no-op stub versions in the #else case, and then call those
+functions unconditionally from .c files.  The compiler will avoid generating
+any code for the stub calls, producing identical results, but the logic will
+remain easy to follow.
+
+Prefer to compile out entire functions, rather than portions of functions or
+portions of expressions.  Rather than putting an ifdef in an expression, factor
+out part or all of the expression into a separate helper function and apply the
+conditional to that function.
+
+If you have a function or variable which may potentially go unused in a
+particular configuration, and the compiler would warn about its definition
+going unused, mark the definition as __maybe_unused rather than wrapping it in
+a preprocessor conditional.  (However, if a function or variable *always* goes
+unused, delete it.)
+
+Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
+symbol into a C boolean expression, and use it in a normal C conditional:
+
+        if (IS_ENABLED(CONFIG_SOMETHING)) {
+                ...
+        }
+
+The compiler will constant-fold the conditional away, and include or exclude
+the block of code just as with an #ifdef, so this will not add any runtime
+overhead.  However, this approach still allows the C compiler to see the code
+inside the block, and check it for correctness (syntax, types, symbol
+references, etc).  Thus, you still have to use an #ifdef if the code inside the
+block references symbols that will not exist if the condition is not met.
+
+At the end of any non-trivial #if or #ifdef block (more than a few lines),
+place a comment after the #endif on the same line, noting the conditional
+expression used.  For instance:
+
+#ifdef CONFIG_SOMETHING
+...
+#endif /* CONFIG_SOMETHING */
+
 
                 Appendix I: References