[PATCH 4/6] x86,kvm: Fix apf_task_wake_one() serialization

[Davidlohr Bueso]

During code inspection, the following potential race was seen:

CPU0                                    CPU1
kvm_async_pf_task_wait                  apf_task_wake_one
  [S] prepare_to_swait(&n.wq)
                                          [L] swait_active(&n->wq)
                                          [S] hlist_del_init(&n->link);
  [L] if (!hlist_unhahed(&n.link))
        schedule()

Properly serialize swait_active() checks such that a wakeup is
not missed.

Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
---
 arch/x86/kernel/kvm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 874827b0d7ca..aa60a08b65b1 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -180,7 +180,7 @@ static void apf_task_wake_one(struct kvm_task_sleep_node *n)
        hlist_del_init(&n->link);
        if (n->halted)
                smp_send_reschedule(n->cpu);
-       else if (swait_active(&n->wq))
+       else if (swq_has_sleeper(&n->wq))
                swake_up(&n->wq);
 }
 
-- 
2.12.0