Re: [LKP] [tty] 0b4f83d510: INFO:task_blocked_for_more_than#seconds

[Dmitry Safonov <dima@arista.com>]

Hi Sergey, Jiri,

On Mon, 2018-09-10 at 14:14 +0900, Sergey Senozhatsky wrote:
> On (09/07/18 08:39), Jiri Slaby wrote:
> > > [  244.944070] 
> > > [  244.944070] Showing all locks held in the system:
> > > [  244.945558] 1 lock held by khungtaskd/18:
> > > [  244.946495]  #0: (____ptrval____) (rcu_read_lock){....}, at:
> > > debug_show_all_locks+0x16/0x190
> > > [  244.948503] 2 locks held by askfirst/235:
> > > [  244.949439]  #0: (____ptrval____) (&tty->ldisc_sem){++++}, at:
> > > tty_ldisc_ref_wait+0x25/0x50
> > > [  244.951762]  #1: (____ptrval____) (&ldata-
> > > >atomic_read_lock){+.+.}, at: n_tty_read+0x13d/0xa00
> > 
> > Here, it just seems to wait for input from the user.
> > 
> > > [  244.953799] 1 lock held by validate_data/655:
> > > [  244.954814]  #0: (____ptrval____) (&tty->ldisc_sem){++++}, at:
> > > tty_ldisc_ref_wait+0x25/0x50
> > > [  244.956764] 1 lock held by dnsmasq/668:
> > > [  244.957649]  #0: (____ptrval____) (&tty->ldisc_sem){++++}, at:
> > > tty_ldisc_ref_wait+0x25/0x50
> > > [  244.959598] 1 lock held by dropbear/734:
> > > [  244.967564]  #0: (____ptrval____) (&tty->ldisc_sem){++++}, at:
> > > tty_ldisc_ref_wait+0x25/0x50
> > 
> > Hmm, I assume there is another task waiting for write_ldsem and
> > that one
> > prevents these readers to proceed. Unfortunately, due to the
> > defunct
> > __ptrval__ pointer hashing crap, we do not see who is waiting for
> > what.
> > But I am guessing all are the same locks.
> 
> Hmm, interesting. Am I getting it right that the test did pass
> before.
> And now we see that sort of/smells like live-lock right after the
> introduction of tty_ldisc_lock() to tty_reopen().
> 
> > So I think, we are forced to limit the waiting to 5 seconds in
> > reopen in
> > the end too (the same as we do for new open in tty_init_dev).
> 
> If I got it right, LKP did test the 5*HZ patch
> 
> 	retval = tty_ldisc_lock(tty, 5 * HZ);
> 
> At least it's
>  In-Reply-To: <20180829022353.23568-3-dima@arista.com>
> 
> and
>  Message-Id: <20180829022353.23568-3-dima@arista.com>
> 
> is the patch which does the 5*HZ lock timeout thing.

Yeah, I also noticed on the weekend that the commit in the mentioned
branch is from v1..

Currently, I tried to reproduce it like ~15-20 times, but unlucky :(

It looks like, the lockup wasn't introduced by this commit, but
unfortunately the commit made it more likely. At least, that's what I
suppose after I've found this report:
https://lkml.org/lkml/2017/11/21/216

It seems to me that the lockup is triggered by:
[  244.948503] 2 locks held by askfirst/235:
[  244.949439]  #0: (____ptrval____) (&tty->ldisc_sem){++++}, at:
tty_ldisc_ref_wait+0x25/0x50
[  244.951762]  #1: (____ptrval____) (&ldata->atomic_read_lock){+.+.},
at: n_tty_read+0x13d/0xa00

Looking into this..

-- 
Thanks,
             Dmitry