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    SubjectRe: [PATCH 24/25] r/o bind mounts: track number of mount writers
    On Thu, 20 Sep 2007 12:53:20 -0700 Dave Hansen <haveblue@us.ibm.com> wrote:

    > This is the real meat of the entire series. It actually
    > implements the tracking of the number of writers to a mount.
    > However, it causes scalability problems because there can
    > be hundreds of cpus doing open()/close() on files on the
    > same mnt at the same time. Even an atomic_t in the mnt
    > has massive scalaing problems because the cacheline gets
    > so terribly contended.
    >
    > This uses a statically-allocated percpu variable. All
    > operations are local to a cpu as long that cpu operates on
    > the same mount, and there are no writer count imbalances.
    > Writer count imbalances happen when a write is taken on one
    > cpu, and released on another, like when an open/close pair
    > is performed on two different cpus because the task moved.


    Did you test with lockdep enabled?

    =============================================
    [ INFO: possible recursive locking detected ]
    2.6.23-rc7-mm1 #1
    ---------------------------------------------
    swapper/1 is trying to acquire lock:
    (&writer->lock){--..}, at: [<c0197a32>] lock_and_coalesce_cpu_mnt_writer_counts+0x32/0x70

    but task is already holding lock:
    (&writer->lock){--..}, at: [<c0197a32>] lock_and_coalesce_cpu_mnt_writer_counts+0x32/0x70

    other info that might help us debug this:
    1 lock held by swapper/1:
    #0: (&writer->lock){--..}, at: [<c0197a32>] lock_and_coalesce_cpu_mnt_writer_counts+0x32/0x70

    stack backtrace:
    [<c0103ffa>] show_trace_log_lvl+0x1a/0x30
    [<c0104b82>] show_trace+0x12/0x20
    [<c0104c96>] dump_stack+0x16/0x20
    [<c0144dc5>] __lock_acquire+0xde5/0x10a0
    [<c01450fa>] lock_acquire+0x7a/0xa0
    [<c03e734c>] _spin_lock+0x2c/0x40
    [<c0197a32>] lock_and_coalesce_cpu_mnt_writer_counts+0x32/0x70
    [<c01982c6>] mntput_no_expire+0x36/0xc0
    [<c0188f15>] path_release_on_umount+0x15/0x20
    [<c0198930>] sys_umount+0x40/0x230
    [<c010070b>] name_to_dev_t+0x9b/0x270
    [<c05230c2>] prepare_namespace+0x62/0x1b0
    [<c05226ca>] kernel_init+0x21a/0x320
    [<c0103b47>] kernel_thread_helper+0x7/0x10
    =======================

    It look like a false positive to me, but really, for a patchset of this
    complexity and maturity I cannot fathom how it could have escaped any
    lockdep testing.

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