[PATCH 4/7] kvm,x86: Fix apf_task_wake_one() wq serialization

[Davidlohr Bueso <dave@stgolabs.net>]

During code inspection, the following potential race was seen:

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

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

Signed-off-by: Davidlohr Bueso <dbueso@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