I agree that when the tty->low_latency flag is set on these machines,
the drivers that were modified by my patch will behave differently:
they will call the flush_to_ldisc() function directly instead of using
the work queue. So that indeed introduces different functionality.

flush_to_ldisc calls into the tty core code which sleeps. The rule is
that you may not call flush_to_ldisc except from a workqueue context.

If you break that rule then various bad things will occur (eg rx ->
flush_to_ldisc -> n_tty -> flow control -> tx -> deadlock) in a lot of
drivers.

But what I don't understand is how this would cause the machine to
crash? Even when the flush_to_ldisc() function is called from hard IRQ
context this would cause no problems: the flush_to_ldisc() function
uses IRQ save spin locks instead of mutexes to protect it's critical
section. Right?

Furthermore the majority of TTY drivers currently already use the
tty_flip_buffer_push() function.

Several of them are ones that shouldn't.

Alan