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From: 520047054719-0001@t-online.de (Oliver Neukum)
Reply-To: Oliver.Neukum@lrz.uni-muenchen.de
To: george anzinger <george@mvista.com>
Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable
Date: Tue, 15 Jan 2002 09:32:17 +0100
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Cc: Momchil Velikov <velco@fadata.bg>, linux-kernel@vger.kernel.org
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On Tuesday 15 January 2002 08:10, george anzinger wrote:

> Yes, this is classic priority inversion.  It is here now, today with
> semaphors which are held by code that blocks.  If the code doesn't
> block, why not use a spin lock?  If it does, well the problem is here

Because eg. other code that holds the semaphore needs to sleep

> now.  I suppose we could set a preempt disable around a semaphore if it
> makes you feel better.  It doesn't fix the problem if the task blocks

It would make me feel better, but it would defeat the purpose.
There's a lot of code holding semaphores.

> AND it is legal to block while holding a preemption lock.

But it's easier to fix. If you can preempt only by explicitely
sleeping, you can beat priority invasion by changing basically
only wake_up. If you can be preempted at random, you need to know
who holds a semaphore.

	Regards
		Oliver
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