linux-kernel - [patch 0/6] futex: Bugfixes and FUTEX_LOCK

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Message-Id: <20210422194417.866740847@linutronix.de>
Date:   Thu, 22 Apr 2021 21:44:17 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Lukasz Majewski <lukma@...x.de>,
        Florian Weimer <fweimer@...hat.com>,
        Carlos O'Donell <carlos@...hat.com>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Ingo Molnar <mingo@...nel.org>,
        Darren Hart <dvhart@...radead.org>,
        Andrei Vagin <avagin@...il.com>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: [patch 0/6] futex: Bugfixes and FUTEX_LOCK_PI2

The following series started off looking into supporting selectable clocks
for FUTEX_LOCK_PI which is hardcoded to CLOCK_REALTIME and cannot be
changed.

On the way I found two bugs related to the timeout handling:

  - The allowance for FUTEX_WAIT to use FUTEX_CLOCK_REALTIME is broken and
    never worked.

  - The recent time namespace support wreckaged FUTEX_LOCK_PI timeouts when
    the task belongs to a namespace which has an CLOCK_MONOTONIC offset.

Both should have been caught by that Gleixner dude when merging them,
but it seems he's getting old.

Not having a selectable clock for PI futexes is inconsistent because all
other interfaces have it. Unfortunately this was figured out by glibc folks
quite some time ago, but nobody told us :(

The nasty hack to support it would be to treat FUTEX_CLOCK_REALTIME inverse
for FUTEX_LOCK_PI, but that's a horrible idea. Adding a new flag to the
futex op, i.e. FUTEX_CLOCK_MONOTONIC would be possible, but that's yet
another variant which makes is harder for libraries to have a consistent
clock selection handling.

So I went the way to let FUTEX_LOCK_PI alone and to add FUTEX_LOCK_PI2
which handles the clocks the same way as the other operands.

Thoughts?

The series is also available from git:

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git futex

Thanks,

	tglx
---
 include/uapi/linux/futex.h |    1 
 kernel/futex.c             |   89 +++++++++++++++++++++++----------------------
 2 files changed, 47 insertions(+), 43 deletions(-)