[PATCH RFC 0/3] nptl: Introduce and use FUTEX_LOCK_PI2

[Kurt Kanzenbach <kurt@linutronix.de>]

Hi,

Linux real time application often make use of mutexes with priority inheritance
enabled due to problems such as unbounded priority inversion. In addition, some
applications make use of timeouts e.g., by utilizing pthread_mutex_clocklock().

However, the combination of priority inheritance enabled mutexes with timeouts
based on CLOCK_MONOTONIC doesn't work. That is because the underlying Linux
kernel futex interface doesn't support it, yet. Using CLOCK_REALTIME is
possible, but it is subject to adjustments (NTP, PTP, ...). Therefore, Thomas
proposed to add a new futex operation called FUTEX_LOCK_PI2 with support for
selectable clocks [1]. That patch series is not merged, yet.

Here is a proof of concept patch set adding FUTEX_LOCK_PI2 in order to support
pthread_mutex_clocklock(MONOTONIC)/PI. The idea is to use futex_lock_pi2() by
default, and fallback to futex_lock_pi() in case the kernel is too old. I think,
you get the idea.

There's also a bugreport for glibc with a test case:

 https://sourceware.org/bugzilla/show_bug.cgi?id=26801

Thoughts?

Thanks,
Kurt

[1] - https://lkml.kernel.org/lkml/20210422194417.866740847@linutronix.de/

Kurt Kanzenbach (3):
  nptl: Introduce futex_lock_pi2()
  nptl: Use futex_lock_pi2()
  nptl: Include CLOCK_MONOTONIC in mutex tests

 nptl/pthread_mutex_timedlock.c            | 24 ++++--
 nptl/tst-mutexpi10.c                      |  8 ++
 sysdeps/nptl/futex-internal.h             | 94 +++++++++++++++++++++++
 sysdeps/nptl/lowlevellock-futex.h         |  1 +
 sysdeps/pthread/tst-mutex5.c              |  3 +-
 sysdeps/pthread/tst-mutex9.c              |  3 +-
 sysdeps/unix/sysv/linux/kernel-features.h |  8 ++
 7 files changed, 133 insertions(+), 8 deletions(-)

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2.30.2