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 14:44:54 -0700
From:   Darren Hart <dvhart@...radead.org>
To:     Andy Lutomirski <luto@...nel.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>,
        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 07:03:53PM +0000, Andy Lutomirski wrote:
> 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.

I dug up the cover letter and got the motivation and withdraw my
objection. I understand the motivation to put the motivation in the
cover letter in a large series, but I think there should have been
something indicating the need for this change in the individual patches,
even just a single line like Andy's first sentence above.

Thanks,

-- 
Darren Hart
VMware Open Source Technology Center