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    SubjectRe: [PATCH v9 01/10] dax: Use percpu rwsem for dax_{read,write}_lock()
    On Sun, Dec 26, 2021 at 6:35 AM Shiyang Ruan <ruansy.fnst@fujitsu.com> wrote:
    >
    > In order to introduce dax holder registration, we need a write lock for
    > dax.

    As far as I can see, no, a write lock is not needed while the holder
    is being registered.

    The synchronization that is needed is to make sure that the device
    stays live over the registration event, and that any in-flight holder
    operations are flushed before the device transitions from live to
    dead, and that in turn relates to the live state of the pgmap.

    The dax device cannot switch from live to dead without first flushing
    all readers, so holding dax_read_lock() over the register holder event
    should be sufficient. If you are worried about 2 or more potential
    holders colliding at registration time, I would expect that's already
    prevented by block device exclusive holder synchronization, but you
    could also use cmpxchg and a single pointer to a 'struct dax_holder {
    void *holder_data, struct dax_holder_operations *holder_ops }'. If you
    are worried about memory_failure triggering while the filesystem is
    shutting down it can do a synchronize_srcu(&dax_srcu) if it really
    needs to ensure that the notify path is idle after removing the holder
    registration.

    ...are there any cases remaining not covered by the above suggestions?

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