On Mon, Mar 20, 2017 at 3:28 AM, Stephen Rothwell [off-list ref] wrote:

quoted

Hi Greg,

Today's linux-next merge of the tty tree got a conflict in:

  drivers/tty/tty_ldisc.c

between commit:

  5362544bebe8 ("tty: don't panic on OOM in tty_set_ldisc()")

from the tty.current tree and commit:

  71472fa9c52b ("tty: Fix ldisc crash on reopened tty")

from the tty tree.

I fixed it up (see below) and can carry the fix as necessary. This
is now fixed as far as linux-next is concerned, but any non trivial
conflicts should be mentioned to your upstream maintainer when your tree
is submitted for merging.  You may also want to consider cooperating
with the maintainer of the conflicting tree to minimise any particularly
complex conflicts.

--
Cheers,
Stephen Rothwell

diff --cc drivers/tty/tty_ldisc.c
index b0500a0a87b8,4ee7742dced3..000000000000

--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c

@@@ -621,14 -669,17 +621,15 @@@ int tty_ldisc_reinit(struct tty_struct
                tty_ldisc_put(tty->ldisc);
        }

-       /* switch the line discipline */
-       tty->ldisc = ld;
        tty_set_termios_ldisc(tty, disc);
-       retval = tty_ldisc_open(tty, tty->ldisc);
+       retval = tty_ldisc_open(tty, ld);
        if (retval) {
-               tty_ldisc_put(tty->ldisc);
-               tty->ldisc = NULL;
 -              if (!WARN_ON(disc == N_TTY)) {
 -                      tty_ldisc_put(ld);
 -                      ld = NULL;
 -              }
++              tty_ldisc_put(ld);
++              ld = NULL;
        }
+
+       /* switch the line discipline */
+       smp_store_release(&tty->ldisc, ld);
        return retval;
  }

Peter,

Looking at your patch "tty: Fix ldisc crash on reopened tty", I think
there is a missed barrier in tty_ldisc_ref. A single barrier does not
have any effect, they always need to be in pairs. So I think we also
need at least:

@@ -295,7 +295,8 @@ struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
        struct tty_ldisc *ld = NULL;

        if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
-               ld = tty->ldisc;
+               ld = READ_ONCE(tty->ldisc);
+               read_barrier_depends();
                if (!ld)
                        ldsem_up_read(&tty->ldisc_sem);
        }

Or simply:

@@ -295,7 +295,8 @@ struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
        struct tty_ldisc *ld = NULL;

        if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
-               ld = tty->ldisc;
+               /* pairs with smp_store_release in tty_ldisc_reinit */
+               ld = smp_load_acquire(&tty->ldisc);
                if (!ld)
                        ldsem_up_read(&tty->ldisc_sem);
        }