tty: Fix hang at ldsem_down_read()

When a controlling tty is being hung up and the hang up is
waiting for a just-signalled tty reader or writer to exit, and a new tty
reader/writer tries to acquire an ldisc reference concurrently with the
ldisc reference release from the signalled reader/writer, the hangup
can hang. The new reader/writer is sleeping in ldsem_down_read() and the
hangup is sleeping in ldsem_down_write() [1].

The new reader/writer fails to wakeup the waiting hangup because the
wrong lock count value is checked (the old lock count rather than the new
lock count) to see if the lock is unowned.

Change helper function to return the new lock count if the cmpxchg was
successful; document this behavior.

[1] edited dmesg log from reporter

SysRq : Show Blocked State
  task                        PC stack   pid father
systemd         D ffff88040c4f0000     0     1      0 0x00000000
 ffff88040c49fbe0 0000000000000046 ffff88040c4a0000 ffff88040c49ffd8
 00000000001d3980 00000000001d3980 ffff88040c4a0000 ffff88040593d840
 ffff88040c49fb40 ffffffff810a4cc0 0000000000000006 0000000000000023
Call Trace:
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff817a6649>] schedule+0x24/0x5e
 [<ffffffff817a588b>] schedule_timeout+0x15b/0x1ec
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff817aa691>] ? _raw_spin_unlock_irq+0x24/0x26
 [<ffffffff817aa10c>] down_read_failed+0xe3/0x1b9
 [<ffffffff817aa26d>] ldsem_down_read+0x8b/0xa5
 [<ffffffff8142b5ca>] ? tty_ldisc_ref_wait+0x1b/0x44
 [<ffffffff8142b5ca>] tty_ldisc_ref_wait+0x1b/0x44
 [<ffffffff81423f5b>] tty_write+0x7d/0x28a
 [<ffffffff814241f5>] redirected_tty_write+0x8d/0x98
 [<ffffffff81424168>] ? tty_write+0x28a/0x28a
 [<ffffffff8115d03f>] do_loop_readv_writev+0x56/0x79
 [<ffffffff8115e604>] do_readv_writev+0x1b0/0x1ff
 [<ffffffff8116ea0b>] ? do_vfs_ioctl+0x32a/0x489
 [<ffffffff81167d9d>] ? final_putname+0x1d/0x3a
 [<ffffffff8115e6c7>] vfs_writev+0x2e/0x49
 [<ffffffff8115e7d3>] SyS_writev+0x47/0xaa
 [<ffffffff817ab822>] system_call_fastpath+0x16/0x1b
bash            D ffffffff81c104c0     0  5469   5302 0x00000082
 ffff8800cf817ac0 0000000000000046 ffff8804086b22a0 ffff8800cf817fd8
 00000000001d3980 00000000001d3980 ffff8804086b22a0 ffff8800cf817a48
 000000000000b9a0 ffff8800cf817a78 ffffffff81004675 ffff8800cf817a44
Call Trace:
 [<ffffffff81004675>] ? dump_trace+0x165/0x29c
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff8100edda>] ? save_stack_trace+0x26/0x41
 [<ffffffff817a6649>] schedule+0x24/0x5e
 [<ffffffff817a588b>] schedule_timeout+0x15b/0x1ec
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff817a9f03>] ? down_write_failed+0xa3/0x1c9
 [<ffffffff817aa691>] ? _raw_spin_unlock_irq+0x24/0x26
 [<ffffffff817a9f0b>] down_write_failed+0xab/0x1c9
 [<ffffffff817aa300>] ldsem_down_write+0x79/0xb1
 [<ffffffff817aada3>] ? tty_ldisc_lock_pair_timeout+0xa5/0xd9
 [<ffffffff817aada3>] tty_ldisc_lock_pair_timeout+0xa5/0xd9
 [<ffffffff8142bf33>] tty_ldisc_hangup+0xc4/0x218
 [<ffffffff81423ab3>] __tty_hangup+0x2e2/0x3ed
 [<ffffffff81424a76>] disassociate_ctty+0x63/0x226
 [<ffffffff81078aa7>] do_exit+0x79f/0xa11
 [<ffffffff81086bdb>] ? get_signal_to_deliver+0x206/0x62f
 [<ffffffff810b4bfb>] ? lock_release_holdtime.part.8+0xf/0x16e
 [<ffffffff81079b05>] do_group_exit+0x47/0xb5
 [<ffffffff81086c16>] get_signal_to_deliver+0x241/0x62f
 [<ffffffff810020a7>] do_signal+0x43/0x59d
 [<ffffffff810f2af7>] ? __audit_syscall_exit+0x21a/0x2a8
 [<ffffffff810b4bfb>] ? lock_release_holdtime.part.8+0xf/0x16e
 [<ffffffff81002655>] do_notify_resume+0x54/0x6c
 [<ffffffff817abaf8>] int_signal+0x12/0x17

Reported-by: Sami Farin <sami.farin@gmail.com>
Cc: <stable@vger.kernel.org> # 3.12.x
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
