From: "Jan Beulich" <JBeulich@suse.com>
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>
Subject: Re: [PATCHv2 3/5] evtchn: use a per-event channel lock for sending events
Date: Wed, 17 Jun 2015 08:05:54 +0100 [thread overview]
Message-ID: <558138720200007800085D74@mail.emea.novell.com> (raw)
In-Reply-To: <5580514A.5000702@citrix.com>
>>> On 16.06.15 at 18:39, <david.vrabel@citrix.com> wrote:
> On 16/06/15 17:19, Jan Beulich wrote:
>>>>> On 16.06.15 at 17:58, <david.vrabel@citrix.com> wrote:
>>> On 16/06/15 16:19, David Vrabel wrote:
>>>>>> @@ -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.
>>>
>>> Uh, no. This is notify. I've kept the locking like this because of the
>>> ld->is_dying check. I think we need the ld->event_lock in case
>>> d->is_dying is set and evtchn_destroy(ld) is called.
>>
>> Right, but if evtchn_destroy() was a concern, then this wouldn't
>> apply just here, but also in the sending path you are relaxing.
>> Afaict due to the channel lock being taken in __evtchn_close()
>> you can drop the even lock here as the latest after you acquired
>> the channel one (I haven't been able to convince myself yet that
>> dropping it even before that would be okay).
>
> But in the evtchn_send() case, we're in a hypercall so we know
> ld->is_dying is false and thus cannot be racing with evtchn_destroy(ld).
That's only the common code path; there's an evtchn_send() call
from __domain_finalise_shutdown() which afaict doesn't make such
guarantees (in particular there clearly are d != current->domain
cases here).
And then - how is being in a hypercall a guard against is_dying not
getting set?
> It would be good to remove event_lock from notify_xen_event_channel() as
> well since this is heavily used for ioreqs and vm events. Let me have a
> more careful look.
Indeed, thanks.
Jan
next prev parent reply other threads:[~2015-06-17 7:05 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-15 15:48 [PATCHv2 0/5] evtchn: Improve scalebility David Vrabel
2015-06-15 15:48 ` [PATCHv2 1/5] evtchn: factor out freeing an event channel David Vrabel
2015-06-15 15:48 ` [PATCHv2 2/5] evtchn: simplify port_is_valid() David Vrabel
2015-06-15 15:57 ` Ian Campbell
2015-06-15 15:59 ` David Vrabel
2015-06-15 16:09 ` Ian Campbell
2015-06-15 15:48 ` [PATCHv2 3/5] evtchn: use a per-event channel lock for sending events David Vrabel
2015-06-16 9:18 ` Jan Beulich
2015-06-16 9:34 ` David Vrabel
2015-06-16 9:51 ` Jan Beulich
2015-06-16 9:57 ` David Vrabel
2015-06-16 15:19 ` David Vrabel
2015-06-16 15:58 ` David Vrabel
2015-06-16 16:19 ` Jan Beulich
2015-06-16 16:39 ` David Vrabel
2015-06-17 7:05 ` Jan Beulich [this message]
2015-06-16 16:06 ` Jan Beulich
2015-06-15 15:48 ` [PATCHv2 4/5] evtchn: remove the locking when unmasking an event channel David Vrabel
2015-06-16 9:19 ` Jan Beulich
2015-06-16 9:40 ` David Vrabel
2015-06-15 15:48 ` [PATCHv2 5/5] evtchn: pad struct evtchn to 64 bytes David Vrabel
2015-06-16 9:22 ` Jan Beulich
2015-06-16 11:14 ` Julien Grall
2015-06-16 11:59 ` Jan Beulich
2015-06-16 12:57 ` Julien Grall
2015-06-16 13:13 ` David Vrabel
2015-06-16 13:27 ` Julien Grall
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