tty: Hold write ldisc sem in tty_reopen()

Hi all,

Here are fixes that worth to have in the @stable, as they were hit by different people, including Arista on v4.9 stable.

And for linux-next - adding lockdep asserts for line discipline changing code, verifying that write ldisc sem will be held forthwith.

Mikulas, can you add your tested-by on this patches set again, please? I tried to reproduce reboot issue on qemu-hppa and even built cross-compiler for pa-risc.. but was unlucky in reproducing.

Thanks,
Dima

Changes since v5:
- Better commit tags
- Hopefully fixed issue with reboot on pa-risc with Debian 5

Changes since v4:
- back to lock ldisc with (5*HZ) timeout in tty_reopen()
(LKP report link: lkml.kernel.org/r/<[email protected]>)
- reordered 3/7 with 2/7 for LKP robot

Changes since v3:
- Added tested-by Mark Rutland (thanks!)
- Dropped patch with smp_wmb() - wrong idea
- lockdep_assert_held() should be actually lockdep_assert_held_exclusive()
- Described why tty_ldisc_open() can be called without ldisc_sem held for pty slave end (o_tty).
- Added Peter's patch for dropping self-made lockdep annotations
- Fix for a reader(s) of ldisc semaphore waiting for an active reader(s)

Changes since v2:
- Added reviewed-by tags
- Hopefully, fixed reported by 0-day issue.
- Added optional fix for wait_readers decrement

Changes since v1:
- Added tested-by/reported-by tags
- Dropped 3/4 (locking tty pair for lockdep sake),
Because of that - not adding lockdep_assert_held() in tty_ldisc_open()
- Added 4/4 cleanup to inc tty->count only on success of tty_ldisc_reinit()
- lock ldisc without (5*HZ) timeout in tty_reopen()

v1 link: lkml.kernel.org/r/<[email protected]> v2 link: lkml.kernel.org/r/<[email protected]> v3 link: lkml.kernel.org/r/<[email protected]> v4 link: lkml.kernel.org/r/<[email protected]>

Cc: Daniel Axtens <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Michael Neuling <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Cc: Nathan March <[email protected]>
Cc: Pasi Kärkkäinen <[email protected]>
Cc: Peter Hurley <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: "Rong, Chen" <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Tan Xiaojun <[email protected]>
Cc: Tetsuo Handa <[email protected]> (please, ignore if I Cc'ed you mistakenly)

Dmitry Safonov (6):
tty/ldsem: Wake up readers after timed out down_write()
tty: Hold tty_ldisc_lock() during tty_reopen()
tty: Don't block on IO when ldisc change is pending
tty: Simplify tty->count math in tty_reopen() tty/ldsem: Add lockdep asserts for ldisc_sem tty/ldsem: Decrement wait_readers on timeouted down_read()

Peter Zijlstra (1):
tty/ldsem: Convert to regular lockdep annotations

 drivers/tty/n_hdlc.c    |  4 +--
 drivers/tty/n_r3964.c   |  2 +-
 drivers/tty/n_tty.c     |  8 +++---
 drivers/tty/tty_io.c    | 14 ++++++----
 drivers/tty/tty_ldisc.c | 16 +++++++++++
 drivers/tty/tty_ldsem.c | 62 +++++++++++++++++------------------------
 include/linux/tty.h     |  7 +++++
 7 files changed, 63 insertions(+), 50 deletions(-)

2.19.1