linux-kernel - Re: [PATCH v2 03/36] mm: use do_futex() instead of sys_futex() in mm

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Date:   Fri, 16 Mar 2018 19:03:53 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Darren Hart <dvhart@...radead.org>
Cc:     Dominik Brodowski <linux@...inikbrodowski.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 03/36] mm: use do_futex() instead of sys_futex() in mm_release()

On Fri, Mar 16, 2018 at 6:43 PM, Darren Hart <dvhart@...radead.org> wrote:
> On Thu, Mar 15, 2018 at 08:04:56PM +0100, Dominik Brodowski wrote:
>> sys_futex() is a wrapper to do_futex() which does not modify any
>> values here:
>>
>> - uaddr, val and val3 are kept the same
>>
>> - op is masked with FUTEX_CMD_MASK, but is always set to FUTEX_WAKE.
>>   Therefore, val2 is always 0.
>>
>> - as utime is set to NULL, *timeout is NULL
>>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Darren Hart <dvhart@...radead.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
>
> Hi Dominik,
>
> I'm missing the "why" part here. What is it you are trying to address?
>
> do_futex is not currently in use outside of the futex implementation,
> while sys_futex is. This decouples the interface from the
> implementation. While this is perhaps less critical within the
> kernel, I don't see a compelling reason to increase the coupling
> between the mm and futex implementations.
>
> Without a compelling WHY, Nack from me.
>

We want to make some changes to the way that the syscall entry code
invokes syscalls, and these changes will make it impossible to call
sys_xyz() functions from the kernel.  So we can make sys_futex() be a
trivial wrapper around a new ksys_futex(), or we can do a patch like
this.