linux-kernel - Re: [patch V4 10/12] futex: Convert to scoped user access

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Message-ID: <20251023221450.1303003d@pumpkin>
Date: Thu, 23 Oct 2025 22:14:50 +0100
From: David Laight <david.laight.linux@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, LKML
 <linux-kernel@...r.kernel.org>, 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, 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, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Cooper
 <andrew.cooper3@...rix.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 V4 10/12] futex: Convert to scoped user access

On Thu, 23 Oct 2025 09:26:12 -1000
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, 23 Oct 2025 at 08:44, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > But as you said out-of-line function call it occured to me that these
> > helpers might be just named get/put_user_inline(). Hmm?  
> 
> Yeah, with a comment that clearly says "you need to have actual
> performance numbers for why this needs to be inlined" for people to
> use it.

Avoiding an extra clac/stac pair for two accesses might be enough.
But for a single access it might be hard to justify.

(Even if 'return' instructions are horribly painful.
Although anyone chasing performance is probably using a local system
and just disables all that 'stuff'.)

> 
>            Linus