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    SubjectRe: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL
    On Sat, 2010-05-08 at 08:27 +1000, Benjamin Herrenschmidt wrote:
    > > Also, to solve this problem when several cpus may spin on the owner,
    > > I wonder adaptive spinning is doing the right thing here. We should
    > > have only one spinner I think, and the rest should go to sleep as the
    > > time to spin on several subsequent owners would be much better gained
    > > to do something else (schedule something else or power saving).
    > > In fact, that too could deserve some tests.
    > Right, the problem is due to the fact that we skip spinning if there's
    > already a waiter but we don't know that there is already a spinner so we
    > can end up with multiple spinners.
    > I don't see a non invasive way to fix that.. we could add a spinner
    > counter to the mutex but that sucks a bit. Might still be worthwhile,
    > not sure. Peter, what do you reckon ?

    If its a large problem the lock is overly contended and _that_ needs
    fixing. I don't at all feel like adding atomic ops to the spin loop to
    try and detect this.

    As to the 2 jiffy spin timeout, I guess we should add a lockdep warning
    for that, because anybody holding a mutex for longer than 2 jiffies and
    not sleeping does need fixing anyway.

     \ /
      Last update: 2010-05-10 09:59    [W:0.024 / U:0.720 seconds]
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