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    SubjectRe: [PATCH] percpu_counter: Fix __percpu_counter_sum()
    On Sun, Dec 07, 2008 at 08:52:50PM -0800, Andrew Morton wrote:
    > The first patch which was added (pre-2.6.27) was "percpu_counter: new
    > function percpu_counter_sum_and_set". This added the broken-by-design
    > percpu_counter_sum_and_set() function, **and used it in ext4**.

    Mea culpa, I was the one who reviewed Mingming's patch, and missed
    this. Part of the problem was that percpu_counter.c isn't well
    documented, and I so saw the spinlock, but didn't realize it only
    protected reference counter, and not the per-cpu array. I should have
    read through code more thoroughly before approving the patch.

    I suppose if we wanted we could add a rw spinlock which mediates
    access to a "foreign" cpu counter (i.e., percpu_counter_add gets a
    shared lock, and percpu_counter_set needs an exclusive lock) but it's
    probably not worth it.

    Actually, if all popular architectures had a hardware-implemented
    atomic_t, I wonder how much ext4 really needs the percpu counter,
    especially given ext4's multiblock allocator; with ext3, given that
    each block allocation required taking a per-filesystem spin lock,
    optimizing away that spinlock was far more important for improving
    ext3's scalability. But with the multiblock allocator, it may that
    we're going through a lot more effort than what is truly necessary.

    - Ted

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