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[PATCH 2.5.66] Real-time futexes (priority inheritance/protection/robust support) take 5

Hi all

More fixes for the real-time futexes. Priority inheritance has now
some more fixes (some of them more or less ugly tweaks) and it
should work better now (some bugs and deadlocks fixed). Still

This is my second try into implementing real-time futex
support. The patch below encloses a proposal on a way to do
real-time futexes that support priority-inheritance,
priority-protection, dead-owner recovery (more flexible than Sun's
Robust Mutex Extension) and dead lock detection, a sample
implementation (sans priority-protection) and a set of test
programs along with a very simple thread library to test it.

Note this is NOT a replacement for futexes. rtfutexes cannot do
things that normal futexes can do because of the constraints that
"real-time-ness" impose. It is intended as a complementary

I have been working on this for a while, and got input from many
different people in different companies who are interested in
these features (eg: Intel, Cisco, Montavista, OSDL ...) We are
also working on a way to blend this into NPTL so it can do

Version 5

- Ported to 2.5.66 - this fixed the timing problems found;
  according to George Anzinger, it was a problem in the timer

- Fix allocation of rtfutex_q: when we drop the spin lock, we need
  to claim again, as somebody else might have preempted and
  modified it. This makes the code much more stable.

- General cleanups (debug, doc) and optimizations (moving
  not-common code paths out of the way - it makes it kind of
  uglier though).

- Fix deadlock caused when rtfutex_do_recycle() runs while the
  same CPU was running with the rtfutex_lock held (I keep stomping
  into this one) - right now just do a trylock and abort if not

- Fail when trying to lock a futex in non recoverable state.

- Fix different aspects in PI (stop the propagation when a
  non-PI/PP futex is hit and fall back to deadlock
  detection). Make it cleaner when we set the priority of a task
  because of inheritance. Force a boosted task to yield when the
  boosting task stops waiting (still needs some work).

- Optimize rtfutex_sign_task() a wee bit for the "all clear" case.

- Allow any kind of task to use rtfutex, even if it is not
  realtime. However, PI/PP requires SCHED_FIFO or SCHED_RR. Use
  static_prio for non-realtime tasks instead of prio.

- Is slow as hell ... I mean, it is expected to be slower than
  normal futexes because they are more complex, but currently it
  sucks. Working on that - mainly removing any usage of
  kmap_atomic(); the TLB flush is a killer.

- Funny behavior on PI with test-11.c; sometimes the thread TL
  overpowers lower-prio TP from the very start.

Version 4

- Currently on 2.5.64

- Does not modify original futexes - a new interface is added


- Priority protection not yet implemented.

- Many fixmes here and there that require answers

- Hook for properly acting when the priority of a waiting task is
  changed not implemented yet. 

- The tweak in timer.c is kind of ugly, and needs to be done much more
  generic; maybe even moved into wake_up_process().

Check out kernel/rtfutex.c for a quick roadmap;
Documentation/rtfutex.txt for a longer series of rants and a
something more like a design reference. Documentation/rtfutex-api.txt 
describes how to call the functions (in the test library,
test/src/include/rtfutex.h is more up-to-date). 

Patch at: http://sost.net/pub/linux/rtfutex-5.patch
Test pkg at: http://sost.net/pub/linux/rtfutex-test-5.tar.gz

Build with:

./configure --with-headers=RTFUTEX-PATCHED-LINUX-TREE/include


Inaky Perez-Gonzalez -- Not speaking for Intel - opinions are my own [or my fault]

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