From: Jakob <jakobkoschel@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Arnd Bergman <arnd@arndb.de>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Kees Cook <keescook@chromium.org>,
Mike Rapoport <rppt@kernel.org>,
"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
Cristiano Giuffrida <c.giuffrida@vu.nl>,
"Bos, H.J." <h.j.bos@vu.nl>
Subject: Re: [RFC PATCH 04/13] vfio/mdev: remove the usage of the list iterator after the loop
Date: Wed, 23 Feb 2022 23:08:53 +0100 [thread overview]
Message-ID: <BA4ACA21-2368-4BEB-ACF0-F4C2042880F4@gmail.com> (raw)
In-Reply-To: <CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com>
> On 23. Feb 2022, at 21:22, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> On Wed, Feb 23, 2022 at 12:15 PM Jakob <jakobkoschel@gmail.com> wrote:
>>
>> in such a case you would still have to set the iterator value to
>> NULL when reaching the terminating condition or am I missing something?
>
> No.
>
> Make the rule be "you never use the iterator outside the loop".
>
> IOW, the code sequence is
>
> some_struct *ptr, *iter;
with C99 iter would be defined within the loop instead right?
>
> ptr = NULL;
> list_for_each_entry(iter, ...) {
> if (iter_matches_condition(iter)) {
> ptr = iter;
> break;
> }
> }
>
> .. never use 'iter' here - you use 'ptr' and check it for NULL ..
>
> See? Same number of variables as using a separate 'bool found' flag,
> but simpler code, and it matches the rule of 'don't use iter outside
> the loop'.
ah yes this does make sense. I missed the part of using a separate
'ptr' variable. Thanks for clarifying.
I think this is a great idea.
There are cases where pos->member is used (the only legitimate way to
use it right now). I suppose those turn into something like this
(this example is inspired by dev_add_offload() (net/core/gro.c:38)):
some_struct *ptr, *iter;
list_head *list_ptr;
ptr = NULL;
list_for_each_entry(iter, head, list) {
if (iter_matches_condition(iter)) {
ptr = iter;
break;
}
}
if (ptr)
list_ptr = head->prev;
else
list_ptr = iter->list.prev;
list_add(..., list_ptr);
before it was simply
list_add(..., iter->list.prev);
The other possibility I suppose would be:
if (!ptr)
ptr = container_of(head, typeof(*ptr), list)
list_add(..., ptr->list.prev);
which leaves you with the same type confusion as before, being far from
ideal.
> This is how you'd have to do it anyway if we start using a C99 style
> 'declare iter _in_ the loop' model.
>
> And as mentioned, it actually tends to lead to better code, since the
> code outside the loop only has one variable live, not two.
>
> Of course, compilers can do a lot of optimizations, so a 'found'
> variable can be made to generate good code too - if the compiler just
> tracks it and notices, and turns the 'break' into a 'goto found', and
> the fallthrough into the 'goto not_found'.
>
> So 'better code generation' is debatable, but even if the compiler can
> do as good a job with a separate 'bool' variable and some cleverness,
> I think we should strive for code where we make it easy for the
> compiler to DTRT - and where the generated code is easier to match up
> with what we wrote.
>
> Linus
If there is interest, I'm happy to send a new patch set once the fixes are clear.
Jakob
next prev parent reply other threads:[~2022-02-23 22:09 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-17 18:48 [RFC PATCH 00/13] Proposal for speculative safe list iterator Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 01/13] list: introduce speculative safe list_for_each_entry() Jakob Koschel
2022-02-17 19:29 ` Greg Kroah-Hartman
2022-02-18 16:29 ` Jann Horn
2022-02-18 16:29 ` Jann Horn
2022-02-23 14:32 ` Jakob
2022-02-19 19:44 ` Jann Horn
2022-02-17 18:48 ` [RFC PATCH 02/13] scripts: coccinelle: adapt to find list_for_each_entry nospec issues Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 03/13] usb: remove the usage of the list iterator after the loop Jakob Koschel
2022-02-17 19:28 ` Linus Torvalds
2022-02-23 14:13 ` Jakob
2022-02-23 14:16 ` Jakob
2022-02-24 10:33 ` Greg Kroah-Hartman
2022-02-24 17:56 ` Linus Torvalds
[not found] ` <6d191223d93249a98511177d4af08420@pexch012b.vu.local>
2022-02-24 10:46 ` Cristiano Giuffrida
2022-02-24 11:26 ` Greg Kroah-Hartman
2022-02-23 18:47 ` Linus Torvalds
2022-02-23 19:23 ` Linus Torvalds
2022-02-23 19:43 ` Linus Torvalds
2022-02-23 20:24 ` Arnd Bergmann
2022-02-23 20:43 ` Linus Torvalds
2022-02-23 20:48 ` Arnd Bergmann
2022-02-23 21:53 ` Linus Torvalds
2022-02-24 16:04 ` Nathan Chancellor
2022-02-23 20:54 ` Linus Torvalds
2022-02-23 22:21 ` David Laight
2022-02-25 21:36 ` Uecker, Martin
2022-02-25 22:02 ` Linus Torvalds
2022-02-26 1:21 ` Martin Uecker
2022-02-27 18:12 ` Miguel Ojeda
2022-02-28 7:08 ` Martin Uecker
2022-02-28 13:49 ` Miguel Ojeda
2022-03-01 20:26 ` Linus Torvalds
2022-03-02 7:27 ` Martin Uecker
2022-02-26 12:42 ` Segher Boessenkool
2022-02-26 22:14 ` Arnd Bergmann
2022-02-26 23:03 ` Linus Torvalds
2022-02-27 1:19 ` Segher Boessenkool
2022-02-27 1:09 ` Segher Boessenkool
2022-02-27 7:10 ` David Laight
2022-02-27 11:32 ` Segher Boessenkool
2022-02-27 18:09 ` Miguel Ojeda
2022-02-27 20:17 ` Segher Boessenkool
2022-02-27 21:04 ` Linus Torvalds
2022-02-28 6:15 ` David Laight
2022-02-27 22:43 ` Miguel Ojeda
2022-02-27 21:28 ` Arnd Bergmann
2022-02-27 22:43 ` Segher Boessenkool
2022-02-17 18:48 ` [RFC PATCH 04/13] vfio/mdev: " Jakob Koschel
2022-02-18 15:12 ` Jason Gunthorpe
2022-02-23 14:18 ` Jakob
2022-02-23 19:06 ` Linus Torvalds
2022-02-23 19:12 ` Jason Gunthorpe
2022-02-23 19:31 ` Linus Torvalds
2022-02-23 20:15 ` Jakob
2022-02-23 20:22 ` Linus Torvalds
2022-02-23 22:08 ` Jakob [this message]
2022-02-23 20:19 ` Rasmus Villemoes
2022-02-23 20:34 ` Linus Torvalds
2022-02-17 18:48 ` [RFC PATCH 05/13] drivers/perf: " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 06/13] ARM: mmp: " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 07/13] udp_tunnel: " Jakob Koschel
2022-02-23 20:00 ` Christophe JAILLET
2022-02-24 6:20 ` Dan Carpenter
2022-02-17 18:48 ` [RFC PATCH 08/13] net: dsa: future proof usage of " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 09/13] drbd: " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 10/13] powerpc/spufs: " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 11/13] ath6kl: remove use " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 12/13] staging: greybus: audio: Remove usage " Jakob Koschel
2022-02-17 18:48 ` [RFC PATCH 13/13] scsi: mpt3sas: comment about invalid usage of the list iterator Jakob Koschel
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=BA4ACA21-2368-4BEB-ACF0-F4C2042880F4@gmail.com \
--to=jakobkoschel@gmail.com \
--cc=akpm@linux-foundation.org \
--cc=andriy.shevchenko@linux.intel.com \
--cc=arnd@arndb.de \
--cc=bjohannesmeyer@gmail.com \
--cc=c.giuffrida@vu.nl \
--cc=gregkh@linuxfoundation.org \
--cc=gustavo@embeddedor.com \
--cc=h.j.bos@vu.nl \
--cc=jgg@ziepe.ca \
--cc=keescook@chromium.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=rppt@kernel.org \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).