On Fri Jun 25 2021, Kurt Kanzenbach wrote:
> Linux real time applications 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 didn't support it, yet. Using CLOCK_REALTIME is possible,
> but it is subject to adjustments (NTP, PTP, ...). Therefore, Thomas proposed and
> introduced a new futex operation called FUTEX_LOCK_PI2 with support for
> selectable clocks [1].
>
> 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.
>
> There's also a bugreport for glibc with a test case:
>
>  https://sourceware.org/bugzilla/show_bug.cgi?id=26801
>
> Changes since RFC:
>
>  * FUTEX_LOCK_PI2 merged in upstream Linux kernel (Thomas Gleixner, Peter Zijlstra)
>  * Make of __ASSUME_FUTEX_LOCK_PI2 in the implementation (Adhemerval Zanella)
>  * Get rid of probing for FUTEX_LOCK_PI2 in timedlock (Adhemerval Zanella)
>  * Adjust the test cases (Adhemerval Zanella)
>  * Use correct Linux kernel version (Joseph Myers)
>
> Previous versions:
>
>  * https://sourceware.org/pipermail/libc-alpha/2021-June/127798.html

Ping. Any comments on this version?

Thanks,
Kurt