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

[Paolo Bonzini]

On 10/09/2017 11:20, Paolo Bonzini wrote:
> On 05/09/2017 21:00, Davidlohr Bueso wrote:
>> 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@xxxxxxx>
>> ---
>>  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);
>>  }
> 
> After Nick's patch, swake_up starts with:
> 
> 	smp_mb();
>         if (!swait_active(q))
>                 return;
> 
> so we can just remove the test here (and in patch 2).
> 
> The other patches could also use a better swait API, for example:
> 
> 1) add a public __swake_up routine that omits the memory barrier, and
> which can be used in patch 3.  Perhaps better: omit the out-of-lock
> check in __swake_up: then the caller can use it if it knows there is a
> waiter.  In those cases the memory barrier is expensive.
> 
> 2) change swake_up and __swake_up to return true if they woke up a
> process (or alternatively 0/-EAGAIN).  Patches 5 and 6 now need not call
> anymore either swq_has_sleepers or swait_active, and that saves a memory
> barrier too.
> 
> What do you think?

... doh, I missed PeterZ's remark that the early test is gone in tip.
Then the series makes total sense.  Peter, if you ack patch 1 I can push
it through the KVM tree.

Paolo