Re: [patch 3/3] [-rt] Fixes the timeout-bug in the rtmutex/PI-futex.

[Esben Nielsen]

On Sun, 23 Jul 2006, Steven Rostedt wrote:

> On Sun, 2006-07-23 at 02:18 +0100, Esben Nielsen wrote:
> > This patch fixes
> >
> > 1) The timeout bug in rtmutexes and PI futexes: When a task is blocked on a
> > lock with timeout and times out it will not wakeup until the owner of the lock
> > is done. This is because the owner is boosted to the same priority as the
> > blocked task and therefore has the CPU such the blocked task never gets around
> > to de-boost it!
> >
> > 2) setscheduler() now does the PI walking - but defers the work to the blocked
> > task.
> >
> > 3) In general it makes sure that a task, which is boosting another has enough
> > priority to do the de-boosting no matter how complicated the lock structure is,
> > or how many times the priorities have changed.
> >
> > The idea behind the patch is simple:
> > If a task is boosting another it is scheduled in LIFO order and it will never
> > loose it's priority. This property lasts until it has left the lock operation
> > (successfully or not).
> >
> > The needed priority to do the unboosting is stored in task->boosting_prio.
> > In the current patch this is always increasing (numerically decreasing) while
> > trying to take a lock. In a future it might be found safe to decrease
> > boosting_prio before finally leaving the lock operation.
> >
> >   include/linux/rtmutex.h                               |    1
> >   include/linux/sched.h                                 |    7
> >   kernel/fork.c                                         |    1
> >   kernel/rtmutex.c                                      |  151 +++++++++++++-----
>
> It is possible that these changes can break the pi code in rtmutex.c.

Hmm, I am quite confident this one doesn't :-) It is tested against my own
user-space unit tester and against Thomas's in-kernel rt-tester. And my 
stress PriorityInheritanceTest by the way.

> I
> haven't analyzed it enough yet.  But just so that you know that your
> changes don't break the code, and to make it easier for me to look at
> it. Please update Documentation/rt-mutex-design.txt including your
> changes.  This will be a good exercise to see if it doesn't really break
> anything, and it will give other reviewers a better starting point for
> review.

Is that up-to-date in the -rt tree? The last patch you sent was to 
2.6.18-rc2, right?

Esben

> >   kernel/rtmutex_common.h                               |    1
> >   kernel/sched.c                                        |   14 +
> >   scripts/rt-tester/t5-l4-pi-boost-deboost-setsched.tst |   42 +++--
> >   scripts/rt-tester/t5-l4-pi-boost-deboost.tst          |   12 -
> >   8 files changed, 167 insertions(+), 62 deletions(-)
>
> -- Steve
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