linux-kernel - [PATCH 4/6] x86,kvm: Fix apf_task_wake

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Message-Id: <20170905190022.1474-5-dave@stgolabs.net>
Date:   Tue,  5 Sep 2017 12:00:20 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     mingo@...hat.com, peterz@...radead.org, pbonzini@...hat.com
Cc:     npiggin@...il.com, paulmck@...ux.vnet.ibm.com, dave@...olabs.net,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 4/6] x86,kvm: Fix apf_task_wake_one() serialization

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 <dbueso@...e.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