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    Subjectaio poll and a new in-kernel poll API V9
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    Hi all,

    this series adds support for the IOCB_CMD_POLL operation to poll for the
    readyness of file descriptors using the aio subsystem. The API is based
    on patches that existed in RHAS2.1 and RHEL3, which means it already is
    supported by libaio. To implement the poll support efficiently new
    methods to poll are introduced in struct file_operations: get_poll_head
    and poll_mask. The first one returns a wait_queue_head to wait on
    (lifetime is bound by the file), and the second does a non-blocking
    check for the POLL* events. This allows aio poll to work without
    any additional context switches, unlike epoll.

    This series sits on top of the aio-fsync series that also includes
    support for io_pgetevents.

    The changes were sponsored by Scylladb, and improve performance
    of the seastar framework up to 10%, while also removing the need
    for a privileged SCHED_FIFO epoll listener thread.

    git://git.infradead.org/users/hch/vfs.git aio-poll.9

    Gitweb:

    http://git.infradead.org/users/hch/vfs.git/shortlog/refs/heads/aio-poll.9

    Libaio changes:

    https://pagure.io/libaio.git io-poll

    Seastar changes (not updated for the new io_pgetevens ABI yet):

    https://github.com/avikivity/seastar/commits/aio

    Changes since V9:
    - make delayed cancellation conditional again
    - add a cancel_kiocb file operation to split delayed vs normal cancel

    Changes since V8:
    - make delayed cancellation safe and unconditional

    Changes since V7:
    - reworked cancellation

    Changes since V6:
    - small changelog updates
    - rebased on top of the aio-fsync changes

    Changes since V4:
    - rebased ontop of Linux 4.16-rc4

    Changes since V3:
    - remove the pre-sleep ->poll_mask call in vfs_poll,
    allow ->get_poll_head to return POLL* values.

    Changes since V2:
    - removed a double initialization
    - new vfs_get_poll_head helper
    - document that ->get_poll_head can return NULL
    - call ->poll_mask before sleeping
    - various ACKs
    - add conversion of random to ->poll_mask
    - add conversion of af_alg to ->poll_mask
    - lacking ->poll_mask support now returns -EINVAL for IOCB_CMD_POLL
    - reshuffled the series so that prep patches and everything not
    requiring the new in-kernel poll API is in the beginning

    Changes since V1:
    - handle the NULL ->poll case in vfs_poll
    - dropped the file argument to the ->poll_mask socket operation
    - replace the ->pre_poll socket operation with ->get_poll_head as
    in the file operations

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