On Sun, Jan 13, 2019 at 08:49:36AM -0800, Matthew Wilcox wrote:

On Thu, Jan 10, 2019 at 11:48:13PM +0000, Jason Gunthorpe wrote:

quoted

+The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
+only evaluate to 0 or 1, and implicit or explicit conversion to bool
+automatically converts the value to true or false. When using bool types the
+!! construction is not needed, which eliminates a class of bugs.
+
+When working with bool values the true and false definitions should be used
+instead of 0 and 1.
+
+bool function return types and stack variables are always fine to use whenever
+appropriate. Use of bool is encouraged to improve readability and is often a
+better option than 'int' for storing boolean values.

It's awkward to start a sentence with a lower case letter.  How about
rephrasing this paragraph and the following one as:

  Using bool as the return type of a function or as a variable is always
  fine when appropriate.  It often improves readability and is a better option
  than int for storing boolean values.  Using bool in data structures is
  more debatable; its size and alignment can vary between architectures.

This is more concise, but I think if the coding style is not going to
give a concrete advise then it should at least provide some general
information so the reader can try and make an informed choice.

That is why I had it expand on some of the rationals a little bit,
along with a concrete direction to not use bool in the cases Linus
specifically called out.

quoted

+Do not use bool if cache line layout or size of the value matters, its size
+and alignment varies based on the compiled architecture. Structures that are
+optimized for alignment and size should not use bool.
+
+If a structure has many true/false values, consider consolidating them into a
+bitfield with 1 bit members, or using an appropriate fixed width type, such as
+u8.

JAson