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SubjectRe: tty->low_latency + irq context
> with tty->low_latency set, but it doesn't AFAICS. One possibility for
> deadlock is if the tty->buf.lock spinlock is taken on behalf of a user
> process...

The case to watch out for is

flip_buffer_push -> ldisc -> driver write of echo/^S/^Q

if you call flip_buffer_push while holding your own lock you may get in
a mess on the echo path.

> * data is received, enough to completely fill the tty buffer
> * tty_flip_buffer_push() schedules flush_to_ldisc()
> * before flush_to_ldisc() runs, more data is received
> * flush_to_ldisc() truncates the incoming data (look for
> tty->receive_room)
> I don't see how this is supposed to work in general.

For non fake tty hardware at real speeds it wasn't a problem under about
1Mbit. Current tty layer code just uses memory buffering based on kmalloc
and has a 64K limit instead. Works better SMP, scales better and we no
longer need to do stunts like the flip buffers to scrape 56Kbit on a

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