Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty_reopen()

From: Dmitry Safonov
Date: Wed Sep 19 2018 - 16:21:59 EST

On Wed, 2018-09-19 at 16:03 -0400, Mikulas Patocka wrote:
> On Wed, 19 Sep 2018, Dmitry Safonov wrote:
> > Thanks much for the testing, Mikulas.
> > Could you try to bisect which of the patches causes it?
> > The most important are 1,2,3 - probably, one of them caused it..
> > I'll stare a bit into the code.
> The patch 3 causes it.
> The hangs during reboot take either 80 seconds, 3 minutes or 3:25.

Thanks much for that again.
Any chance you have sysrq enabled and could print: locks held in system
and kernel backtraces for applications?

I suppose, userspace hangs in n_tty_receive_buf_common(), which is
better of cause than null-ptr dereference, but I believe we can improve
it by stopping a reader earlier if there is any writer pending.