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

Alan Cox
2009-08-21T18:42:18Z

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

It needs the hardware active and running to do the block_til_ready

> 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 */

If the first user succeeded then second should open immediately as the use
count is >= 1 already. It is re-opening the open device, unless the
hangup occurs first.

>
> 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?

Usual paths

open
block_til_ready
open returns 0
do stuff
close

open
block_til_ready
open returns 0
do stuff
hangup (closes hw side)
further stuff errors
close (does nothing to the hw side tty_port_close knows about
this)

open
block_til_ready
hangup
open returns error
close path called - knows about hangup in tty_close_port

Hence the reason it always calls the close path to cleanup hardware and
other resources even if open returns an error

>
> 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?

The tty structure is the virtual interface side, the tty_port is the
hardware device. Historically the port side varied randomly by device
preventing any commonality being extracted into helpers.

For most hardware the tty_port lifetime is the physical port lifetime,
the tty lifetime is open->final close (or a final kref drop afterwards)

It also means eventually that we can have a tty_port and things hung off
it for all hardware which means

- Most of the locking costs for receiving v hangup go away (as you
queue to the tty_port not the tty - ie move the tty_buffer struct)
- It becomes possible to have a proper generic console/debug console
without hacks and fake tty devices.

Most drivers tend to look like

open
test ASYNC_INITIALIZED
init hardware
[either in full or clean up partial]
set ASYNC_INITIALIZED)
any other alloc/counter magic
tty->private_data = my stuff
block_til_ready
return ok/error

close
if (hung_up)
return
if (tty->driver_data == NULL)
return
counts
clean up resources
if (last && test_clear INITIALIZED)
deinit-hardware
return ok/error

hangup
if (initialized & test_clear INITIALIZED) {
deinit hardware
}

which is where I was trying to get the USB code.