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    SubjectRe: [PATCH -v7][RFC]: mutex: implement adaptive spinning


    On Thu, 8 Jan 2009, Steven Rostedt wrote:
    >
    > We keep spinning if the owner changes.

    I think we want to - if you have multiple CPU's and a heavily contended
    lock that acts as a spinlock, we still _do_ want to keep spinning even if
    another CPU gets the lock.

    And I don't even believe that is the bug. I suspect the bug is simpler.

    I think the "need_resched()" needs to go in the outer loop, or at least
    happen in the "!owner" case. Because at least with preemption, what can
    happen otherwise is

    - process A gets the lock, but gets preempted before it sets lock->owner.

    End result: count = 0, owner = NULL.

    - processes B/C goes into the spin loop, filling up all CPU's (assuming
    dual-core here), and will now both loop forever if they hold the kernel
    lock (or have some other preemption disabling thing over their down()).

    And all the while, process A would _happily_ set ->owner, and eventually
    release the mutex, but it never gets to run to do either of them so.

    In fact, you might not even need a process C: all you need is for B to be
    on the same runqueue as A, and having enough load on the other CPU's that
    A never gets migrated away. So "C" might be in user space.

    I dunno. There are probably variations on the above.

    Linus


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