linux-kernel - Re: [RFC PATCH 03/13] usb: remove the usage of the list iterator after the loop

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Message-ID: <20220227224352.GA614@gate.crashing.org>
Date:   Sun, 27 Feb 2022 16:43:52 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jakob <jakobkoschel@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Mike Rapoport <rppt@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [RFC PATCH 03/13] usb: remove the usage of the list iterator after the loop

On Sun, Feb 27, 2022 at 10:28:41PM +0100, Arnd Bergmann wrote:
> On Sun, Feb 27, 2022 at 2:09 AM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
> >
> > So imo we should just never do this by default, not just if the nasty
> > -fwrapv or nastier -fno-strict-overflow is used, just like we suggest
> > in our own documentation.  The only valid reason -Wshift-negative-value
> > is in -Wextra is it warns for situations that always are undefined
> > behaviour (even if not in GCC).
> 
> Ok, I just realized that this is specific to the i915 driver because
> that, unlike
> most of the kernel builds with -Wextra by default. -Wextra is enabled when
> users ask for a 'make W=1' build in linux, and i915 is one of just three
> drivers that enable an equivalent set of warnings, the other ones
> being greybus and btrfs.
> 
> This means to work around the extra warnings, we also just need to disable
> it in the W=1 part of scripts/Makefile.extrawarn, as well as the three drivers
> that copy those options, but not the default warnings that don't include them.

Ah good, all of the workaround in one simple place, neat.

> > Could you open a GCC PR for this?  The current situation is quite
> > suboptimal, and what we document as our implementation choice is much
> > more useful!
> 
> I hope I managed to capture the issue in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104711

That looks fine.  Thank you!

(I attached the testcase to the bug itself, we prefer it that way, maybe
godbolt will go away some day, who knows.)


Segher