From: Linus Torvalds <torvalds@linux-foundation.org>
To: Alexey Dobriyan <adobriyan@gmail.com>,
Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Sparse Mailing-list <linux-sparse@vger.kernel.org>
Subject: Re: [PATCH 00/11] pragma once: treewide conversion
Date: Sun, 28 Feb 2021 09:46:17 -0800 [thread overview]
Message-ID: <CAHk-=wjFWZMVWTbvUMVxQqGKvGMC_BNrahCtTkpEjxoC0k-T=A@mail.gmail.com> (raw)
In-Reply-To: <YDvLYzsGu+l1pQ2y@localhost.localdomain>
On Sun, Feb 28, 2021 at 8:57 AM Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> This is bulk deletion of preprocessor include guards and conversion
> to #pragma once directive.
So as mentioned earlier, I'm not 100% convinced about the advantage of
#pragma once.
But I decided to actually test it, and it turns out that it causes
problems for at least sparse.
Sparse *does* support pragma once, but it does it purely based on
pathname equality. So a simple test-program like this:
File 'pragma.h':
#pragma once
#include "header.h"
works fine. But this doesn't work at all:
#pragma once
#include "./header.h"
because it causes the filename to be different every time, and you
eventually end up with trying to open "././....../pragma.h" and it
causes ENAMETOOLONG.
So at least sparse isn't ready for this.
I guess sparse could always simplify the name, but that's non-trivial.
And honestly, using st_dev/st_ino is problematic too, since
(a) they can easily be re-used for generated files
(b) you'd have to actually open/fstat the filename to use it, which
obviates one of the optimizations
Trying the same on gcc, you don't get that endless "add "./" behavior"
that sparse did, but a quick test shows that it actually opens the
file and reads it three times: once for "pramga.h", once for
"./pragma.h" and a third time for "pragma.h". It only seems to
_expand_ it once, though.
I have no idea what gcc does. Maybe it does some "different name, so
let's open and read it, and then does st_dev/st_ino again". But if so,
why the _third_ time? Is it some guard against "st_ino might have been
re-used, so I'll open the original name and re-verify"?
End result: #pragma is fundamentally less reliable than the
traditional #ifdef guard. The #ifdef guard works fine even if you
re-read the file for whatever reason, while #pragma relies on some
kind of magical behavior.
I'm adding Luc in case he has any ideas of what the magical behavior might be.
Honestly, I think #pragma once is complete garbage. It's really is
fundamenetally more complicated than the #ifdef guard, and it has
absolutely zero upsides.
I'm not taking this pramga series unless somebody can explain why it's
a good thing. Because all I see are downsides.
Linus
next prev parent reply other threads:[~2021-02-28 17:47 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-28 16:57 [PATCH 00/11] pragma once: treewide conversion Alexey Dobriyan
2021-02-28 16:58 ` [PATCH 01/11] pragma once: delete include/linux/atm_suni.h Alexey Dobriyan
2021-02-28 19:05 ` Jakub Kicinski
2021-02-28 16:59 ` [PATCH 02/11] pragma once: convert arch/arm/tools/gen-mach-types Alexey Dobriyan
2021-03-01 10:19 ` Russell King - ARM Linux admin
2021-03-02 15:15 ` Alexey Dobriyan
2021-02-28 16:59 ` [PATCH 03/11] pragma once: convert arch/s390/tools/gen_facilities.c Alexey Dobriyan
2021-02-28 17:00 ` [PATCH 04/11] pragma once: convert drivers/gpu/drm/pl111/pl111_nomadik.h Alexey Dobriyan
2021-03-01 14:41 ` Linus Walleij
2021-02-28 17:01 ` [PATCH 05/11] pragma once: convert drivers/scsi/qla2xxx/qla_target.h Alexey Dobriyan
2021-02-28 22:07 ` Bart Van Assche
2021-02-28 17:02 ` [PATCH 06/11] pragma once: convert include/linux/cb710.h Alexey Dobriyan
2021-03-03 23:13 ` Michał Mirosław
2021-02-28 17:02 ` [PATCH 07/11] pragma once: convert kernel/time/timeconst.bc Alexey Dobriyan
2021-02-28 17:03 ` [PATCH 08/11] pragma once: convert scripts/atomic/ Alexey Dobriyan
2021-03-01 7:55 ` Peter Zijlstra
2021-02-28 17:04 ` [PATCH 09/11] pragma once: convert scripts/selinux/genheaders/genheaders.c Alexey Dobriyan
2021-02-28 18:37 ` Paul Moore
2021-02-28 18:57 ` Alexey Dobriyan
2021-02-28 17:05 ` [PATCH 10/11] pragma once: delete few backslashes Alexey Dobriyan
2021-03-01 8:54 ` Ido Schimmel
2021-03-02 19:00 ` Vineet Gupta
2021-03-04 14:22 ` Edward Cree
2021-03-23 10:09 ` Pavel Machek
2021-02-28 17:05 ` [PATCH 11/11] pragma once: conversion script (in Python 2) Alexey Dobriyan
2021-02-28 17:11 ` [PATCH 12/11] pragma once: scripted treewide conversion Alexey Dobriyan
2021-03-01 17:35 ` Darrick J. Wong
2021-02-28 17:46 ` Linus Torvalds [this message]
2021-02-28 19:34 ` [PATCH 00/11] pragma once: " Alexey Dobriyan
2021-02-28 20:00 ` Linus Torvalds
[not found] ` <877dmo10m3.fsf@tromey.com>
2021-03-03 20:17 ` Linus Torvalds
2021-03-04 13:55 ` David Laight
2021-03-04 20:16 ` Linus Torvalds
2021-03-05 9:19 ` David Laight
2021-03-05 21:23 ` Linus Torvalds
2021-03-06 13:07 ` Miguel Ojeda
2021-03-06 21:33 ` Linus Torvalds
2021-03-23 10:03 ` Pavel Machek
2021-03-01 0:29 ` Luc Van Oostenryck
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