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Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable
From: Robert Love <rml@tech9.net>
To: Oliver.Neukum@lrz.uni-muenchen.de
Cc: Momchil Velikov <velco@fadata.bg>, yodaiken@fsmlabs.com,
        Daniel Phillips <phillips@bonn-fries.net>,
        Roman Zippel <zippel@linux-m68k.org>, linux-kernel@vger.kernel.org
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On Mon, 2002-01-14 at 15:22, Oliver Neukum wrote:

> > No, this isn't needed.  This same problem would occur without
> > preemption.  Our semaphores now have locking rules such that we aren't
> > going to have blatant priority inversion like this (1 holds A needs B, 2
> > holds B needs A).
> 
> No this is a good old deadlock.
> The problem with preemption and SCHED_FIFO is, that due to SCHED_FIFO
> you have no guarantee that any task will make any progress at all.
> Thus a semaphore could basically be held forever.
> That can happen without preemption only if you do something that
> might block.

Well, semaphores block.  And we have these races right now with
SCHED_FIFO tasks.  I still contend preempt does not change the nature of
the problem and it certainly doesn't introduce a new one.

	Robert Love

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