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====== Pthread Condvars: Posix Compliance and the PI gap ======

Tuesday, October 11, 2016 • 16:40 - 17:30

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===== Abstract =====
The pthread condvar uses internal locking which is not configurable for PI. Using
condvars in real-time applications can therefor lead to non-intuitive unbounded
priority inversions, despite the use of a PI mutex. Patches to glibc have provided
non-POSIX (-np) extensions to the condvar implementation, allowing the condvar to be
configured for PI. While these patches are broadly used with the PREEMPT_RT Linux
kernel, they have not yet made it into the upstream glibc sources.

Concurrently, the glibc implementation of condvars was found to be non-compliant with
POSIX, which has lead to the development of a new condvar algorithm, which makes it
impossible to support PI using the existing operations. While a solution has not yet
been identified, there are possibilities involving modifying existing operation or
introducing new PI futex operation which merit discussion.

===== Audience =====
We are soliciting input on implementation from kernel developers with experience in
core locking mechanisms (futex, rt-mutex, etc.) as well as system library developers,
and real-time applications developers. The focus of the presentation is discussion of
potential paths to a solution which is POSIX compliant for the core and supports
Priority Inheritance.

===== Biography =====

==== Darren Hart, Intel Open Source Technology Center ====

Darren's focus is Intel Architecture in embedded, real-time, and safety critical
applications. He is the Linux kernel maintainer for x86 platform drivers and represents
Intel to the LF Real-Time Linux Project, and the OSADL SIL2LinuxMP project. He
previously served as the MinnowBoard program architect for 4 years, providing technical
technical direction to the original MinnowBoard, MAX, and Turbot boards. As the Yocto
Project BSP technical lead, he redesigned the  BSPs for Intel processors and wrote the
manual for Yocto Project kernel development. Prior to Intel, Darren led the Real-Time
Linux development team at IBM for 6 years where he focused on real-time scheduling and
IPC mechanisms.

==== Torvald Riegel, Red Hat ====

Torvald works for Red Hat's Platform Toolchain Team, focusing on
parallelism and concurrency. Recently, he has been revising the
implementations of most of the POSIX Threads synchronization data
structures in glibc and contributed to the improvement of the futex
manpages. He holds a PhD for his work on shared-memory synchronization.

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<WRAP rightalign> Go to [[realtime:events:rt-summit2016:schedule|Real-Time Summit 2016 Schedule]]</WRAP>
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