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

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Date:   Sat, 17 Mar 2018 17:39:37 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Darren Hart <dvhart@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 02:44:54PM -0700, Darren Hart wrote:
> 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.

It's two lines, but I added

	This patch is part of a series which tries to remove in-kernel calls to
	syscalls. On this basis, the syscall entry path can be streamlined.

to all commits in these series which remove in-kernel calls to syscalls.

Thanks,
	Dominik