KVM: fix the race while wakeup all pv guest

In kvm_async_pf_wakeup_all(), we add a dummy apf to vcpu->async_pf.done
without holding vcpu->async_pf.lock, it will break if we are handling apfs
at this time.

Also use 'list_empty_careful()' instead of 'list_empty()'
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

virt/kvm/async_pf.c

@@ -196,7 +196,7 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
 {
 	struct kvm_async_pf *work;
 
-	if (!list_empty(&vcpu->async_pf.done))
+	if (!list_empty_careful(&vcpu->async_pf.done))
 		return 0;
 
 	work = kmem_cache_zalloc(async_pf_cache, GFP_ATOMIC);
@@ -207,7 +207,10 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
 	get_page(bad_page);
 	INIT_LIST_HEAD(&work->queue); /* for list_del to work */
 
+	spin_lock(&vcpu->async_pf.lock);
 	list_add_tail(&work->link, &vcpu->async_pf.done);
+	spin_unlock(&vcpu->async_pf.lock);
+
 	vcpu->async_pf.queued++;
 	return 0;
 }