Re: duplicate read/write locks in net/pfil.c and netinet/ip_fw2.c

From: Stephan Uphoff (ups_at_tree.com)
Date: 08/21/05

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    To: Luigi Rizzo <rizzo@icir.org>
Date: Sat, 20 Aug 2005 18:39:49 -0400

    On Thu, 2005-08-18 at 10:31, Luigi Rizzo wrote:
    > On Thu, Aug 18, 2005 at 10:18:33AM -0400, Stephan Uphoff wrote:
    > > On Thu, 2005-08-18 at 03:57, Luigi Rizzo wrote:
    > ...
    > > > In fact i don't understand why you consider spinning and sleeping
    > > > on a mutex two different things.
    > >
    > > The major difference between sleeping (cv_wait,msleep,..) and blocking
    > > on a mutex is priority inheritance.
    > > If you need to be able to use (non-spin) mutexes while holding a
    > > [R|W]LOCK and use a [R|W]LOCK while holding a (non-spin) mutex then you
    > > need to implement priority inheritance for [R|W]LOCKs.
    >
    > is that required (in FreeBSD, i mean) for algorithmic
    > correctness or just for performance ?

    Hi Luigi,

    It is theoretically required since otherwise low priority user threads
    (programs) could block system (interrupt) threads indefinitely long.

    Example:
            Extreme low priority (nice?) thread A holds read/write
            lock RW as reader
            Thread B is holding mutex M tries to acquire read/write lock
            RW as writer and sleeps.
            Thread C with better priority than A runs and enters a
            busy loop (in user space).
            Interrupt thread preempts C and tries to acquire Mutex M.
            Interrupt priority is propagated to B BUT NOT TO A.
            Interrupt thread blocks on Mutex M.
            Thread C resumes and will block thread I forever if it
            can keep a better priority than thread A.

    In practice you would probably just see bad latency every now and then
    and may never encounter a hang.

    Stephan

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