Re: [bisected] Re: tty lockdep trace

[Mike Galbraith]

On Sun, 2017-06-04 at 10:32 +0200, Greg Kroah-Hartman wrote:
> On Sat, Jun 03, 2017 at 08:33:52AM +0200, Mike Galbraith wrote:
> > On Wed, 2017-05-31 at 13:21 -0400, Dave Jones wrote:
> > > Just hit this during a trinity run.
> > 
> > 925bb1ce47f429f69aad35876df7ecd8c53deb7e is the first bad commit
> > commit 925bb1ce47f429f69aad35876df7ecd8c53deb7e
> > Author: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> > Date:   Thu May 11 12:18:52 2017 +0200
> > 
> >     tty: fix port buffer locking
> 
> Now reverting this.  Oops, sorry, forgot to add Dave and your names to
> the patch revert.  The list of people who reported this was really long,
> many thanks for this.

If flush_to_ldisc() is the problem, and taking atomic_write_lock in
that path an acceptable solution, how about do that a bit differently
instead. ÂLockdep stopped grumbling, vbox seems happy.

925bb1ce47f4 (tty: fix port buffer locking) upset lockdep by holding buf->lock
while acquiring tty->atomic_write_lock.  Move acquisition to flush_to_ldisc(),
taking it prior to taking buf->lock.  Costs a reference, but appeases lockdep.

Not-so-signed-off-by: /me
---
 drivers/tty/tty_buffer.c |   10 ++++++++++
 drivers/tty/tty_port.c   |    2 --
 2 files changed, 10 insertions(+), 2 deletions(-)

--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -465,7 +465,13 @@ static void flush_to_ldisc(struct work_s
 {
 	struct tty_port *port = container_of(work, struct tty_port, buf.work);
 	struct tty_bufhead *buf = &port->buf;
+	struct tty_struct *tty = READ_ONCE(port->itty);
+	struct tty_ldisc *disc = NULL;
 
+	if (tty)
+		disc = tty_ldisc_ref(tty);
+	if (disc)
+		mutex_lock(&tty->atomic_write_lock);
 	mutex_lock(&buf->lock);
 
 	while (1) {
@@ -501,6 +507,10 @@ static void flush_to_ldisc(struct work_s
 	}
 
 	mutex_unlock(&buf->lock);
+	if (disc) {
+		mutex_unlock(&tty->atomic_write_lock);
+		tty_ldisc_deref(disc);
+	}
 
 }
 
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -34,9 +34,7 @@ static int tty_port_default_receive_buf(
 	if (!disc)
 		return 0;
 
-	mutex_lock(&tty->atomic_write_lock);
 	ret = tty_ldisc_receive_buf(disc, p, (char *)f, count);
-	mutex_unlock(&tty->atomic_write_lock);
 
 	tty_ldisc_deref(disc);