tty/ldsem: Wake up readers after timed out down_write()

ldsem_down_read() will sleep if there is pending writer in the queue.
If the writer times out, readers in the queue should be woken up,
otherwise they may miss a chance to acquire the semaphore until the last
active reader will do ldsem_up_read().

There was a couple of reports where there was one active reader and
other readers soft locked up:
  Showing all locks held in the system:
  2 locks held by khungtaskd/17:
   #0:  (rcu_read_lock){......}, at: watchdog+0x124/0x6d1
   #1:  (tasklist_lock){.+.+..}, at: debug_show_all_locks+0x72/0x2d3
  2 locks held by askfirst/123:
   #0:  (&tty->ldisc_sem){.+.+.+}, at: ldsem_down_read+0x46/0x58
   #1:  (&ldata->atomic_read_lock){+.+...}, at: n_tty_read+0x115/0xbe4

Prevent readers wait for active readers to release ldisc semaphore.

Link: lkml.kernel.org/r/20171121132855.ajdv4k6swzhvktl6@wfg-t540p.sh.intel.com
Link: lkml.kernel.org/r/20180907045041.GF1110@shao2-debian
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_ldsem.c

@@ -293,6 +293,16 @@ down_write_failed(struct ld_semaphore *sem, long count, long timeout)
 	if (!locked)
 		atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
 	list_del(&waiter.list);
+
+	/*
+	 * In case of timeout, wake up every reader who gave the right of way
+	 * to writer. Prevent separation readers into two groups:
+	 * one that helds semaphore and another that sleeps.
+	 * (in case of no contention with a writer)
+	 */
+	if (!locked && list_empty(&sem->write_wait))
+		__ldsem_wake_readers(sem);
+
 	raw_spin_unlock_irq(&sem->wait_lock);
 
 	__set_current_state(TASK_RUNNING);