Date: Wed, 24 Nov 2004 04:42:57 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Esben Nielsen <simlo@phys.au.dk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Priority Inheritance Test (Real-Time Preemption)
Message-ID: <20041124034257.GA12571@elte.hu>
References: <20041123133456.GA10453@elte.hu> <Pine.OSF.4.05.10411232343010.4816-100000@da410.ifa.au.dk>
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* Esben Nielsen <simlo@phys.au.dk> wrote:

> I have an idea about what the error(s) is(are): In rt.c policy ==
> SCHED_NORMAL tasks are threaded specially. A task boosted into the
> real-time realm by mutex_setprio() is _still_ SCHED_NORMAL and do not
> gain all the privileges of a real-time task. I suggest that the tests
> on SCHED_NORMAL are replaced by using the rt_task() test which just
> looks at the current priority and thus also would be true on tasks
> temporarely boosted in the real-time realm. [...]

ah, indeed. I thought i fixed all these places but you are right that
there's one more instance remaining, in pick_new_owner():

+               if (w->task->policy == SCHED_NORMAL)
+                       break;

> [...] Another thing: A SCHED_NORMAL task will not be added to the
> pi_waiters list, but it ought to be when it is later boosted into the
> real-time realm. [...]

ok, this is an important one too. Fixing this bug will complicate
pi_setprio() some more, but should be pretty straightforward: the task
that is boosted can be blocked on a single rt-mutex at most.

i'll fix these bugs in the next release. Your systematic testing of PI
is very useful, it already uncovered _tons_ of PI bugs that would be
quite hard to find via normal testing.

> [...] Also, you ignore all tasks being SCHED_NORMAL in the tail of the
> wait list when you try to find the next owner: It could be that one of
> those is boosted.

correct.

> --- linux-2.6.10-rc2-mm2-V0.7.30-9/drivers/char/blocker.c.orig  2004-11-23 20:18:28.000000000 +0100
> +++ linux-2.6.10-rc2-mm2-V0.7.30-9/drivers/char/blocker.c       2004-11-23 20:41:57.742899751 +0100

thx, i'll add this too. Do you have uploaded the matching pi_test
userspace changes too? (url?)

	Ingo
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