uprobes: Fix the waitqueue_active() check in xol_free_insn_slot()

The xol_free_insn_slot()->waitqueue_active() check is buggy. We
need mb() after we set the conditon for wait_event(), or
xol_take_insn_slot() can miss the wakeup.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Pratyush Anand <panand@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150721134036.GA4799@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/uprobes.c

@@ -1337,6 +1337,7 @@ static void xol_free_insn_slot(struct task_struct *tsk)
 
 		clear_bit(slot_nr, area->bitmap);
 		atomic_dec(&area->slot_count);
+		smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
 		if (waitqueue_active(&area->wq))
 			wake_up(&area->wq);