From: Benjamin Herrenschmidt <benh@kernel.crashing.org>

I falled again on the crash in con_do_write() with driver->data beeing
NULL.  It happens during boot, when userland is playing open/close games
with tty's, I was intentionally typing keys like mad during boot trying to
trigger another problem when this one poped up.

Looking at the code, I'm not sure how protected we are by the above (tty)
layer, paulus told me to not rely on anything like locking coming from
there, so I decided to extend the scope of the console semaphore one more
bit to cover races between calls to con_open, con_close and con_write.
Note that in con_do_write, I intentionally drop the semaphore to avoid
keeping it held when waiting on the local buffer, and I added some sanity
checks on tty->driver_data with some printk's in case we still have an open
race by the tty layer.  At least, now, the couple vc_allocated &
tty->driver_data should be protected though.