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    SubjectRe: kernel BUG at mm/truncate.c:475!
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    On Mon, 2010-12-13 at 23:31 -0800, Hugh Dickins wrote:
    > > > + clear_bit_unlock(AS_UNMAPPING, &mapping->flags);
    > > > + smp_mb__after_clear_bit();
    > > > + wake_up_bit(&mapping->flags, AS_UNMAPPING);
    > > > +
    > >
    > > I do think this was premature optimisation. The open-coded lock is
    > > hidden from lockdep so we won't find out if this introduces potential
    > > deadlocks. It would be better to add a new mutex at least temporarily,
    > > then look at replacing it with a MiklosLock later on, when the code is
    > > bedded in.
    > >
    > > At which time, replacing mutexes with MiklosLocks becomes part of a
    > > general "shrink the address_space" exercise in which there's no reason
    > > to exclusively concentrate on that new mutex!
    >
    > Yes, I very much agree with you there: valiant effort by Miklos to
    > avoid bloat, but we're better off using a known primitive for now.

    Also, bit-spinlocks _suck_.. They're not fair, they're expensive and
    like already noted they're hidden from lockdep.

    Ideally we should be removing bit-spinlocks from the kernel, not add
    more.


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