linux-kernel - Re: [PATCH RESEND v4 0/7] futex: Create set_robust

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Message-ID: <fd2b5bb2-590a-47ee-8e56-965a1d09b2fc@igalia.com>
Date: Wed, 18 Jun 2025 13:39:46 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Darren Hart <dvhart@...radead.org>,
 Davidlohr Bueso <dave@...olabs.net>, Shuah Khan <shuah@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, Waiman Long <longman@...hat.com>,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-api@...r.kernel.org, kernel-dev@...lia.com
Subject: Re: [PATCH RESEND v4 0/7] futex: Create set_robust_list2

Hi Sebastian,

Thanks for the feedback!

Em 18/06/2025 04:08, Sebastian Andrzej Siewior escreveu:
> On 2025-06-17 15:34:17 [-0300], André Almeida wrote:
>> This patch adds a new robust_list() syscall. The current syscall
>> can't be expanded to cover the following use case, so a new one is
>> needed. This new syscall allows users to set multiple robust lists per
>> process and to have either 32bit or 64bit pointers in the list.
> 
> Thank you for the reminder. It was on my list, it slipped. Two
> questions:
> - there was a bot warning for v3 but this v4 is a RESEND. It the warning
>    addressed in any way?
> 

Ops, I forgot to address them. I will do it for v5.

> - You say 64bit x86-64 does not have the problem due the compat syscall.
>    Arm64 has this problem. New arm64 do not provide arm32 facility. You
>    introduce the syscall here. Why not introduce the compat syscall
>    instead? I'm sorry if this has been answered somewhere below but this
>    was one question I had while I initially skimmed over the patches.
> 

The main target for this new syscall is Arm64, that can't handle 32 
pointers in the current syscall, so this new interface allows the robust 
list mechanism to know if it needs to do 64 or 32 bit pointer arithmetic 
operations to walk in the list.

Introducing a compat syscall won't fix this, giving that it only works 
in x86-64. We need an entry point for Arm64 that can handle 32 bit pointers.

I hope that it's clear now, let me know if you have more questions :)

> Sebastian