Re: [Qemu-devel] [RFC 3/5] nbd: Use aio_set_fd_handler2()

[Stefan Hajnoczi]

On Sat, May 31, 2014 at 08:43:10PM +0200, Max Reitz wrote:
> Instead of using the main loop function qemu_set_fd_handler2(), use the
> AIO function in the context of the exported BDS.

Managing fd handlers shouldn't be necessary at the NBD code level.  The
current NBD code hasn't been fully converted to coroutines.

This email explains how the NBD code can be fully converted to
coroutines.  It should simplify the code and reduce the chance of bugs.
Whether you want to actually do the conversion is up to you, since it's
somewhat orthogonal to the purpose of this patch series.

The point of coroutines is that blocking operations like send/recv on a
socket should look like regular blocking calls.  Let coroutines handle
the event loop housekeeping (registering fd handlers, callbacks).  Only
use aio explicitly when concurrency is needed.

Here is how I would structure NBD using coroutines:

1 coroutine per connection to receive NBD commands and submit I/O
requests:

def nbd_server_receive_co(conn):
    while True:
        req = nbd_read_req(conn)
    if req is None:
        break
    if req.type == NBD_READ:
        bdrv_aio_readv(bs, ...)
    elif req.type == NBD_WRITE:
        ...

Notice that bdrv_aio_*() is used since we want concurrent I/O requests.

1 coroutine per connection to send NBD replies:

def nbd_server_send_co(conn):
    while True:
        while conn.send_queue:
            resp = conn.send_queue.pop()
            nbd_write_resp(conn, resp)
        qemu_coroutine_yield()

And finally the bdrv_aio_*() callback to put responses on to the send
queue:

def nbd_server_aio_cb(conn):
    resp = NBDResponse(...)
    conn.send_queue.push(resp)
    conn.send_co.enter()

Why is this design cleaner?  Because NBD code doesn't have to worry
about fd handlers.  It uses straightforward coroutine send/recv for
socket I/O inside nbd_read_req() and nbd_write_resp().  It's easy to see
that only one coroutine receives from the socket and that only one
coroutine writes to the socket.

Stefan