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

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Date:	Sun, 23 Jul 2006 17:58:49 +0100 (BST)
From:	Esben Nielsen <nielsen.esben@...glemail.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch 3/3] [-rt] Fixes the timeout-bug in the rtmutex/PI-futex.



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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