Re: [PATCH v2] ext4: Enable code path when DX_DEBUG is set

From: "Theodore Ts'o" <tytso@mit.edu>
To: Vinicius Tinti <viniciustinti@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH v2] ext4: Enable code path when DX_DEBUG is set
Date: Mon, 1 Feb 2021 16:09:48 -0500	[thread overview]
Message-ID: <YBhuHJgZ3QPqHheV@mit.edu> (raw)
In-Reply-To: <CALD9WKzO53AXQW-qQ82VZ41H5=cGdLTUiEoz3X6BmPkb6XaTag@mail.gmail.com>

On Mon, Feb 01, 2021 at 03:41:50PM -0300, Vinicius Tinti wrote:
> 
> My goal is to avoid having a dead code. Three options come to mind.
> 
> The first would be to add another #ifdef SOMETHING (suggest a name).
> But this doesn't remove the code and someone could enable it by accident.

I *really* don't see the point of having the compiler whine about
"dead code", so I'm not terribly fond of
-Wunreachable-code-aggressive.  There may be times where depending on
how things are compiled, we *want* the compiler to remove code block,
and it makes the code less ugly than having #ifdef ... #endif breaking
up the code.

If turning that on requires uglifying many places in the kernel code,
maybe the right answer is... don't.

That being said, I have no problem of replacing

	if (0) {
		...
	}

with

#ifdef DX_DEBUG
	...
#endif

In this particular place.

But before we go there, I want to register my extreme skepticsm about
-Wunreachable-code-aggressive.  How much other places where it is
***obvious*** that the maintainer really knew what they are doing, and
it's just the compiler whining about a false positive?

> > However, if there *is* a bug, having an early detection that the
> > representation invariant of the data structure has been violated can
> > be useful in root causing a bug.  This would probably be clearer if
> > the code was pulled out into a separate function with comments
> > explaining that this is a rep invariant check.
> 
> Good idea. I will do that too.

If you want to do that, and do something like

#ifdef DX_DEBUG
static inline htree_rep_invariant_Check(...)
{
	...
}
#else
static inline htree_rep_invariant_check(...) { }
#endif

I'm not going to complain.  That's actually a better way to go, since
there may be other places in the code where a developer might want to
introduce a rep invariant check.  So that's actually improving the
code, as opposed to making a pointless change just to suppress a
compiler warning.

Of course, then someone will try enabling a -W flag which causes the
compiler to whine about empty function bodies....

					- Ted