linux-kernel - Re: [patch V5 10/12] futex: Convert to get/put_user

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Message-ID: <87frb3uijw.ffs@tglx>
Date: Tue, 28 Oct 2025 16:56:03 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Darren Hart
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, André Almeida
 <andrealmeid@...lia.com>, kernel test robot <lkp@...el.com>, Russell King
 <linux@...linux.org.uk>, linux-arm-kernel@...ts.infradead.org, Linus
 Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org, Madhavan
 Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>, Christophe Leroy
 <christophe.leroy@...roup.eu>, linuxppc-dev@...ts.ozlabs.org, Paul
 Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
 linux-riscv@...ts.infradead.org, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>, Sven Schnelle
 <svens@...ux.ibm.com>, linux-s390@...r.kernel.org, Andrew Cooper
 <andrew.cooper3@...rix.com>, David Laight <david.laight.linux@...il.com>,
 Julia Lawall <Julia.Lawall@...ia.fr>, Nicolas Palix
 <nicolas.palix@...g.fr>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org
Subject: Re: [patch V5 10/12] futex: Convert to get/put_user_inline()

On Tue, Oct 28 2025 at 10:24, Mathieu Desnoyers wrote:
> On 2025-10-27 04:44, Thomas Gleixner wrote:
>> From: Thomas Gleixner <tglx@...utronix.de>
>> 
>> Replace the open coded implementation with the new get/put_user_inline()
>> helpers. This might be replaced by a regular get/put_user(), but that needs
>> a proper performance evaluation.
>
> I understand that this is aiming to keep the same underlying code,
> but I find it surprising that the first user of the "inline" get/put
> user puts the burden of the proof on moving this to regular
> get/put_user() rather than on using the inlined version.
>
> The comment above the inline API clearly states that performance
> numbers are needed to justify the use of inline, not the opposite.
>
> I am concerned that this creates a precedent that may be used by future
> users of the inline API to use it without performance numbers
> justification.

There was not justification for the open coded inline either and
converting it to get/put must be a completely seperate change.