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    SubjectRe: 2.6.31-rc5 regression: Oops when USB Serial disconnected while in use
    On Fri, 21 Aug 2009, Alan Cox wrote:

    > > Alan, related to this problem is the fact that usb_serial_driver
    > > doesn't include a "hangup" method. I'm not sure that making
    > > serial_hangup() call serial_do_down() is correct, but if it is then
    > > shouldn't we call port->serial->type->open() afterwards? Otherwise the
    > > lower driver can't know that the port is still open.
    >
    > After a hangup the port isn't open (at the physical level anyway). It may
    > be re-opened by another open() call later

    Or, oddly enough, another open() call earlier...

    > but the original user lost
    > access to it and shouldn't touch the h/w post hangup completion.

    Then surely serial_open() should call serial->type->open() _after_
    tty_port_block_til_ready(), not before.

    > > Furthermore, shouldn't we check ASYNCB_INITIALIZED in serial_close()?
    >
    > Possibly - I was in the middle of debugging the newest bits when Linus
    > annoyed me. Certainly there are issues in the current code if you get
    > a hangup while waiting for open to complete. I fixed that in the patches
    > I sent Greg but didn't finish debugging it.

    Well, yes. The way it is now, you get:

    serial_open() /* first user */
    serial->type->open() /* initializes the hardware */
    tty_port_block_til_ready() /* returns immediately */

    serial_open() /* second user */
    serial->type->open skipped because the port
    is already open
    tty_port_block_til_ready() /* blocks */

    serial_hangup() /* first connection drops */
    serial_do_down()
    serial->type-close() /* resets the hardware */

    ... tty_port_block_til_ready() returns

    Now the second user tries to do stuff but the hardware isn't ready.
    Instead this should go:

    serial_open() /* first user */
    tty_port_block_til_ready() /* returns immediately */
    serial->type->open() /* initializes the hardware */

    serial_open() /* second user */
    tty_port_block_til_ready() /* blocks */

    serial_hangup() /* first connection drops */
    serial_do_down()
    serial->type-close() /* resets the hardware */

    ... tty_port_block_til_ready() returns
    serial->type->open() /* not skipped */

    Now the second user can proceed to use the hardware. Or have I
    misunderstood how this is intended to work?

    Alan Stern

    P.S.: Can you explain the reason why tty_port and tty_struct are two
    separate structures? Isn't the same port always associated with the
    same tty?



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