Re: mutex_lock_slowpath warning using mxser tty driver (input/output error)

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Apr 02, 2009 at 11:55:08PM +0100, Alan Cox wrote:
> On Thu, 02 Apr 2009 22:23:57 +0200
> Jiri Slaby <jirislaby@gmail.com> wrote:
> 
> > On 04/02/2009 08:44 PM, Alan Cox wrote:
> > >> It appears to want to lock a mutex in interrupt context, something
> > >> that's a definite no-no.
> > >>
> > >> CCed the folks who might know a thing or two about the driver.
> > > 
> > > Moxa is not a USB driver so the USB trace at the end makes no sense at
> > > all.
> > 
> > Yes, unreliable stack entries, but echo_set_canon_col is called from irq
> > context and locks mutex.
> 
> Which means that either the driver is calling the ldisc receive handlers
> from an IRQ directly (not allowed) or calling the tty_flip_buffer_push
> paths and set tty->low_latency
> 
> 2.6.29 is the first kernel that catches this bug with warnings.

It sets tty->low_latency = 1 on ->open(), unconditionally, and does
tty_flip_buffer_push() from the interrupt handler.

Both things seem to have been there for a very long time (git-blame
says longer than git).