Re: 2.6.31-rc5 regression: Oops when USB Serial disconnected while in use

Alan Stern
2009-08-21T17:39:54Z

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?