docs: deprecated.rst: Add uninitialized_var()

Nothing should be using this macro, and the entire idea of tricking the
compiler into silencing such warnings is a mistake.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Joe Perches <joe@perches.com>
Cc: linux-doc@vger.kernel.org
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

Documentation/process/deprecated.rst

@@ -51,6 +51,24 @@ to make sure their systems do not continue running in the face of
 "unreachable" conditions. (For example, see commits like `this one
 <https://git.kernel.org/linus/d4689846881d160a4d12a514e991a740bcb5d65a>`_.)
 
+uninitialized_var()
+-------------------
+For any compiler warnings about uninitialized variables, just add
+an initializer. Using the uninitialized_var() macro (or similar
+warning-silencing tricks) is dangerous as it papers over `real bugs
+<https://lore.kernel.org/lkml/20200603174714.192027-1-glider@google.com/>`_
+(or can in the future), and suppresses unrelated compiler warnings
+(e.g. "unused variable"). If the compiler thinks it is uninitialized,
+either simply initialize the variable or make compiler changes. Keep in
+mind that in most cases, if an initialization is obviously redundant,
+the compiler's dead-store elimination pass will make sure there are no
+needless variable writes.
+
+As Linus has said, this macro
+`must <https://lore.kernel.org/lkml/CA+55aFw+Vbj0i=1TGqCR5vQkCzWJ0QxK6CernOU6eedsudAixw@mail.gmail.com/>`_
+`be <https://lore.kernel.org/lkml/CA+55aFwgbgqhbp1fkxvRKEpzyR5J8n1vKT1VZdz9knmPuXhOeg@mail.gmail.com/>`_
+`removed <https://lore.kernel.org/lkml/CA+55aFz2500WfbKXAx8s67wrm9=yVJu65TpLgN_ybYNv0VEOKA@mail.gmail.com/>`_.
+
 open-coded arithmetic in allocator arguments
 --------------------------------------------
 Dynamic size calculations (especially multiplication) should not be