lkml.org 
[lkml]   [2009]   [Aug]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: WARNING at: drivers/char/tty_ldisc.c


On Sun, 2 Aug 2009, Sergey Senozhatsky wrote:
>
> non-SMP system 'fails' as well.

Ahh, can you trigger this reliably? Is it 100% of the time when you shut
down from single user mode? Or just occasionally?

> > The ldisc refcounts are simply done wrong. They are more debugging aids
> > (for the case where no races occur), than actual memory management
> > refcounts.
>
> tty_ldisc.c:798 tty_ldisc_hangup
> WARN_ON(tty_ldisc_wait_idle(tty) != 0);
>
> gave WARN_ON traces.

Yes, good catch. It means that somebody seems to have held on to the
refcount for more than five seconds.

Which shouldn't happen under any normal situation.

> So, it seems refcount is wrong before
> tty_ldisc_halt(tty);
> tty_ldisc_wait_idle(tty);

Agreed. Or something is just holding the refcount for too long, possibly
due to some deadlockish scenario (ie we migth be in "tty_ldisc_flush()",
and blocked forever on ld->ops->flush_buffer() while holding the ldisc
refcount. And we hold that whole &tty->ldisc_mutex _while_ waiting, so I
can easily see things being blocked on each other.

I'd like to drop the ldisc_mutex while sleeping, but we can't. Not every
caller even holds it. But just for a broken test, can you try the appended
patch (NOT meant for serious consumption!) to see if it migth be a
deadlock (broken by the timeout) on that semaphore?

I take it that you can't get a trace with sysrq-T because nothing gets
logged, and you don't have a serial port console? That would likely
pinpoint it pretty quickly (you could make the WARN_ON() do a
"show_state()" instead - no need to actually physically press 'sysrq-t').

Linus
---
drivers/char/tty_ldisc.c | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
index acd76b7..eb44c45 100644
--- a/drivers/char/tty_ldisc.c
+++ b/drivers/char/tty_ldisc.c
@@ -795,7 +795,9 @@ void tty_ldisc_hangup(struct tty_struct *tty)
if (tty->ldisc) { /* Not yet closed */
/* Switch back to N_TTY */
tty_ldisc_halt(tty);
+ mutex_unlock(&tty->ldisc_mutex); // HACK
tty_ldisc_wait_idle(tty);
+ mutex_lock(&tty->ldisc_mutex); // HACK
tty_ldisc_reinit(tty);
/* At this point we have a closed ldisc and we want to
reopen it. We could defer this to the next open but

\
 
 \ /
  Last update: 2009-10-18 23:28    [W:0.063 / U:33.916 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site