Re: [2.4.17/18pre] VM and swap - it's really unusable

Robert Love
2002-01-14T20:35:12Z

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