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    SubjectRe: RT patch acceptance
    Andi Kleen wrote:
    > What I dislike with RT mutexes is that they convert all locks.
    > It doesnt make much sense to me to have a complex lock that
    > only protects a few lines of code (and a lot of the spinlock
    > code is like this). That is just a waste of cycles.

    I had brought this up in the dim past in the context
    of adaptive mutexes which could via heuristics decide
    whether to spin/sleep.

    > But I always though we should have a new lock type that is between
    > spinlocks and semaphores and is less heavyweight than a semaphore
    > (which tends to be quite slow due to its many context switches). Something
    > like a spinaphore, although it probably doesnt need full semaphore
    > semantics (rarely any code in the kernel uses that anyways). It could
    > spin for a short time and then sleep.

    Spin if the lock is contended and the owner is active
    on a cpu under the assumption the lock owner's average
    hold time is less than that of a context switch. There
    are restrictions as once a path holds an adaptive
    mutex as a spin lock it cannot acquire another adaptive
    mutex as a blocking lock.

    > If you drop irq threads then you cannot convert all locks
    > anymore or have to add ugly in_interrupt()checks. So any conversion like
    > that requires converting locks.

    Yes, I was trying to make that point in an earlier thread.

    -john


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    john.cooper@timesys.com
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