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 <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Pratyush Anand <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2d5b7bd337a7c0867d8a985d11f1e45a18b00c37..4e5e9798aa0c0d426962642b69985b9eb09021d6 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/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);