Re: tty crash in tty_ldisc_receive_buf()

[Benjamin Herrenschmidt]

On Thu, 2017-04-06 at 17:04 +1000, Michael Neuling wrote:
> We see this usually on boot but can also see it if we kill a getty attached to
> tty (which is then respawned by systemd). It seems like we are flushing data to
> a tty at the same time as it's being torn down and restarted.
> 
> I did try the below patch which avoids the crash but locks up one of the CPUs. I
> guess the data never gets flushed if we say nothing is processed.
> 
> This is on powerpc but has also been reported by parisc.
> 
> I'm not at all familiar with the tty layer and looking at the locks, mutexes,
> semaphores and reference counting in there scares the hell out of me.Â
> 
> If anyone has an idea, I'm happy to try a patch.

Note that we noticed one path that called reinit without the ldisc lock
held for writing, we added that, but it didn't fix the problem.

Cheers,
Ben.