kbuild: Remove CONFIG_DEBUG_SECTION_MISMATCH

Modpost's section mismatch detection warns when a non-init function
references an __init function.  The warning is needed because __init
memory is freed during boot, so the non-init function might end up
calling into some random memory.

CONFIG_DEBUG_SECTION_MISMATCH is intended to root out more of these
issues by adding the -fno-inline-functions-called-once compiler flag,
which forces once-called static functions to not be inlined.  As
described in the option's help description:

  - Add the option -fno-inline-functions-called-once to gcc commands.
    When inlining a function annotated with __init in a non-init
    function, we would lose the section information and thus
    the analysis would not catch the illegal reference.
    This option tells gcc to inline less (but it does result in
    a larger kernel).

It's true that if an __init function is inlined in a non-init function,
it's no longer __init.  But that's not actually a problem.

If an inlined __init function doesn't access any other __init functions
or data, the inlining is harmless.

If the inlined __init function does access other __init functions/data,
__init relocations will be generated for a non-init section, and
modpost's section mismatch detection will discover it.

So the option isn't actually useful.  It's not even useful for finding
issues on other configs, which might have different inlining decisions,
because with -O2, once-called static functions are always inlined.  And
anyway, nowadays we have build bots everywhere, which are great at
finding such edge cases.

I get the feeling this option has been responsible for dozens of
"convert inline to __always_inline" patches over the years.  Such
patches usually complain about the compiler's inlining decisions being
unpredictable.  Turns out the culprit is this
CONFIG_DEBUG_SECTION_MISMATCH option, at least for once-called
functions.

This option really has no benefits, and only drawbacks, causing needless
churn.  Remove it.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
7 files changed