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

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