linux-kernel - Re: [RFC] treewide: cleanup unreachable breaks

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Message-ID: <CAKwvOdkR_Ttfo7_JKUiZFVqr=Uh=4b05KCPCSuzwk=zaWtA2_Q@mail.gmail.com>
Date:   Mon, 19 Oct 2020 12:42:15 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Tom Rix <trix@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-edac@...r.kernel.org,
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        clang-built-linux <clang-built-linux@...glegroups.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        George Burgess <gbiv@...gle.com>
Subject: Re: [RFC] treewide: cleanup unreachable breaks

On Sat, Oct 17, 2020 at 10:43 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Sat, Oct 17, 2020 at 09:09:28AM -0700, trix@...hat.com wrote:
> > From: Tom Rix <trix@...hat.com>
> >
> > This is a upcoming change to clean up a new warning treewide.
> > I am wondering if the change could be one mega patch (see below) or
> > normal patch per file about 100 patches or somewhere half way by collecting
> > early acks.
>
> Please break it up into one-patch-per-subsystem, like normal, and get it
> merged that way.
>
> Sending us a patch, without even a diffstat to review, isn't going to
> get you very far...

Tom,
If you're able to automate this cleanup, I suggest checking in a
script that can be run on a directory.  Then for each subsystem you
can say in your commit "I ran scripts/fix_whatever.py on this subdir."
 Then others can help you drive the tree wide cleanup.  Then we can
enable -Wunreachable-code-break either by default, or W=2 right now
might be a good idea.

Ah, George (gbiv@, cc'ed), did an analysis recently of
`-Wunreachable-code-loop-increment`, `-Wunreachable-code-break`, and
`-Wunreachable-code-return` for Android userspace.  From the review:
```
Spoilers: of these, it seems useful to turn on
-Wunreachable-code-loop-increment and -Wunreachable-code-return by
default for Android
...
While these conventions about always having break arguably became
obsolete when we enabled -Wfallthrough, my sample turned up zero
potential bugs caught by this warning, and we'd need to put a lot of
effort into getting a clean tree. So this warning doesn't seem to be
worth it.
```
Looks like there's an order of magnitude of `-Wunreachable-code-break`
than the other two.

We probably should add all 3 to W=2 builds (wrapped in cc-option).
I've filed https://github.com/ClangBuiltLinux/linux/issues/1180 to
follow up on.
-- 
Thanks,
~Nick Desaulniers