From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jan Beulich" <JBeulich@suse.com>
Subject: Re: [PATCHv2 3/5] evtchn: use a per-event channel lock
 for sending events
Date: Tue, 16 Jun 2015 17:06:45 +0100
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To: David Vrabel <david.vrabel@citrix.com>
Cc: xen-devel@lists.xenproject.org, Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Tim Deegan <tim@xen.org>
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>>> On 16.06.15 at 17:19, <david.vrabel@citrix.com> wrote:
> On 16/06/15 10:18, Jan Beulich wrote:
>>>>> On 15.06.15 at 17:48, <david.vrabel@citrix.com> wrote:
>>> @@ -1163,11 +1213,15 @@ int alloc_unbound_xen_event_channel(
>>>      if ( rc )
>>>          goto out;
>>>  
>>> +    spin_lock(&chn->lock);
>>> +
>>>      chn->state = ECS_UNBOUND;
>>>      chn->xen_consumer = get_xen_consumer(notification_fn);
>>>      chn->notify_vcpu_id = lvcpu;
>>>      chn->u.unbound.remote_domid = remote_domid;
>>>  
>>> +    spin_unlock(&chn->lock);
>>> +
>>>   out:
>>>      spin_unlock(&ld->event_lock);
>> 
>> I don't see why you shouldn't be able to move up this unlock.
> 
> Because we need to (also) hold ld->event_lock while changing the state
> from ECS_FREE or a concurrent get_free_port() will find this port again.

I buy this one (and moving the unlock up after the state adjustment
is unlikely to be worth it), but ...

>>> @@ -1221,6 +1277,8 @@ void notify_via_xen_event_channel(struct domain *ld, int lport)
>>>          evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn);
>>>      }
>>>  
>>> +    spin_unlock(&lchn->lock);
>>> +
>>>      spin_unlock(&ld->event_lock);
>>>  }
>> 
>> Again I think the event lock can be dropped earlier now.
> 
> Ditto.

... there's no state change involved here.

Jan