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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jakob <jakobkoschel@gmail.com>
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 12:22:08 -0800	[thread overview]
Message-ID: <CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com> (raw)
In-Reply-To: <F9A66F89-66C2-4322-808B-275384C4CC9D@gmail.com>

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;

        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'.

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

  reply	other threads:[~2022-02-23 20:22 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 [this message]
2022-02-23 22:08                 ` Jakob
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

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