[PATCH 3/3] uprobes: fix the waitqueue_active() check in xol_free_insn_slot()

[Oleg Nesterov <oleg@redhat.com>]

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>
---
 kernel/events/uprobes.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 6a9c5e0..9a4f9df 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1336,6 +1336,7 @@ 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);
 
-- 
1.5.5.1