linux-kernel - Re: [PATCH 2/6] list: add new MACROs to make iterator invisiable outside the loop

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Message-ID: <CAHk-=wi58pvQhMX2sRt7nKqwHAFAmn27MrJg3XbeJgio6ONgdA@mail.gmail.com>
Date:   Fri, 11 Mar 2022 10:41:06 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Xiaomeng Tong <xiam0nd.tong@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jakob Koschel <jakobkoschel@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/6] list: add new MACROs to make iterator invisiable
 outside the loop

On Fri, Mar 11, 2022 at 6:27 AM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> It is possible simply to use spelling to help uncover errors in
> list_traverse()?

I'd love to, and thought that would be a lovely idea, but in another
thread ("") Barnabás Pőcze pointed out that we actually have a fair
number of cases where the list member entries are embedded in internal
structures and have a '.' in them:

  https://lore.kernel.org/all/wKlkWvCGvBrBjshT6gHT23JY9kWImhFPmTKfZWtN5Bkv_OtIFHTy7thr5SAEL6sYDthMDth-rvFETX-gCZPPCb9t2bO1zilj0Q-OTTSbe00=@protonmail.com/

which means that you can't actually append the target_member name
except in the simplest cases, because it wouldn't result in one single
identifier.

Otherwise it would be a lovely idea.

> For architectures without HAVE_LD_DEAD_CODE_DATA_ELIMINATION then the
> "obvious" extension of list_traversal_head() ends up occupying bss
> space. Even replacing the pointer with a zero length array is still
> provoking gcc-11 (arm64) to allocate a byte from bss (often with a lot
> of padding added).

I think compilers give objects at least one byte of space, so that two
different objects get different addresses, and don't compare equal.

That said, I'm not seeing your issue. list_traversal_head() is a
union, and always has that 'struct list_head' in it, and that's the
biggest part of the union.

IOW, the other parts are (a) never used for anything but their type
and (b) will not take up any new space that isn't already used by the
list_head itself.

                  Linus