From: David Vrabel <david.vrabel@citrix.com>
To: Jan Beulich <JBeulich@suse.com>, 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: [PATCHv3 5/6] evtchn: remove the locking when unmasking an event channel
Date: Thu, 18 Jun 2015 13:17:38 +0100 [thread overview]
Message-ID: <5582B6E2.8030001@citrix.com> (raw)
In-Reply-To: <5582D0F902000078000869BE@mail.emea.novell.com>
On 18/06/15 13:08, Jan Beulich wrote:
>>>> On 18.06.15 at 13:36, <david.vrabel@citrix.com> wrote:
>> On 18/06/15 12:30, Jan Beulich wrote:
>>>>>> On 17.06.15 at 14:03, <david.vrabel@citrix.com> wrote:
>>>> --- a/xen/common/event_channel.c
>>>> +++ b/xen/common/event_channel.c
>>>> @@ -978,8 +978,6 @@ int evtchn_unmask(unsigned int port)
>>>> struct domain *d = current->domain;
>>>> struct evtchn *evtchn;
>>>>
>>>> - ASSERT(spin_is_locked(&d->event_lock));
>>>> -
>>>> if ( unlikely(!port_is_valid(d, port)) )
>>>> return -EINVAL;
>>>>
>>>> @@ -1146,9 +1144,7 @@ long do_event_channel_op(int cmd,
>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>> struct evtchn_unmask unmask;
>>>> if ( copy_from_guest(&unmask, arg, 1) != 0 )
>>>> return -EFAULT;
>>>> - spin_lock(¤t->domain->event_lock);
>>>> rc = evtchn_unmask(unmask.port);
>>>> - spin_unlock(¤t->domain->event_lock);
>>>
>>> And, looking particularly at evtchn_fifo_unmask() (and its descendant
>>> evtchn_fifo_set_pending()), you get away without acquiring the port
>>> lock in or around evtchn_port_unmask()? If indeed so, this one would
>>> again be independent on 1, 2, and 4, i.e. could go in together with 3.
>>
>> Yes. This is only dependent on #3 (simplify port_is_valid()).
>
> I'm still not convinced that not taking the port lock is correct: It
> is my understanding that e.g. the (last_vcpu_id,last_priority) pair
> needs to be updated atomically. Yet nothing prevents the
> (notify_vcpu_id,priority) pair now from getting updated in the
> middle of it being snapshot in evtchn_fifo_set_pending(). Are you
> saying this is no problem?
This is serialized by the q lock.
evtchn_fifo_set_pending() has never been serialized by the event lock
because concurrent evtchn_send() calls with two interdomain channels
from different domains would have taken different event locks.
David
next prev parent reply other threads:[~2015-06-18 12:17 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-17 12:02 [PATCHv3 0/6] evtchn: Improve scalebility David Vrabel
2015-06-17 12:02 ` [PATCHv3 1/6] evtchn: clear xen_consumer when clearing state David Vrabel
2015-06-18 10:30 ` Jan Beulich
2015-06-17 12:02 ` [PATCHv3 2/6] evtchn: defer freeing struct evtchn's until evtchn_destroy_final() David Vrabel
2015-06-18 10:36 ` Jan Beulich
2015-06-18 10:40 ` David Vrabel
2015-06-18 11:01 ` Jan Beulich
2015-06-19 9:29 ` Jan Beulich
2015-06-19 9:52 ` David Vrabel
2015-06-19 10:55 ` Jan Beulich
2015-06-19 12:23 ` David Vrabel
2015-06-19 13:04 ` Jan Beulich
2015-06-19 16:58 ` David Vrabel
2015-06-17 12:03 ` [PATCHv3 3/6] evtchn: simplify port_is_valid() David Vrabel
2015-06-17 12:03 ` [PATCHv3 4/6] evtchn: use a per-event channel lock for sending events David Vrabel
2015-06-18 11:20 ` Jan Beulich
2015-06-18 11:39 ` David Vrabel
2015-06-17 12:03 ` [PATCHv3 5/6] evtchn: remove the locking when unmasking an event channel David Vrabel
2015-06-18 11:30 ` Jan Beulich
2015-06-18 11:36 ` David Vrabel
2015-06-18 12:08 ` Jan Beulich
2015-06-18 12:17 ` David Vrabel [this message]
2015-06-17 12:03 ` [PATCHv3 6/6] evtchn: pad struct evtchn to 64 bytes David Vrabel
2015-06-18 11:31 ` Jan Beulich
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