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

[Mikulas Patocka]

On Wed, 19 Sep 2018, Dmitry Safonov wrote:

> 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?

Stacktrace doesn't work on parisc. It used to work on 4.18, but the kernel
4.19 reportedly requires patched binutils.

Mikulas

> 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.
> 
> -- 
> Thanks,
>              Dmitry
>