Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held

From: Julien Grall <julien@xen.org>
To: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Tamas K Lengyel <lengyelt@ainfosec.com>,
	Petre Ovidiu PIRCALABU <ppircalabu@bitdefender.com>,
	Alexandru Isaila <aisaila@bitdefender.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held
Date: Fri, 4 Dec 2020 11:51:37 +0000	[thread overview]
Message-ID: <02a2b77f-27a9-b1b6-1acf-1f136cffdf30@xen.org> (raw)
In-Reply-To: <269f9a2d-7a8d-cba2-801f-6d3b12f9455f@suse.com>

On 04/12/2020 11:48, Jan Beulich wrote:
> On 04.12.2020 12:28, Julien Grall wrote:
>> Hi Jan,
>>
>> On 03/12/2020 10:09, Jan Beulich wrote:
>>> On 02.12.2020 22:10, Julien Grall wrote:
>>>> On 23/11/2020 13:30, Jan Beulich wrote:
>>>>> While there don't look to be any problems with this right now, the lock
>>>>> order implications from holding the lock can be very difficult to follow
>>>>> (and may be easy to violate unknowingly). The present callbacks don't
>>>>> (and no such callback should) have any need for the lock to be held.
>>>>>
>>>>> However, vm_event_disable() frees the structures used by respective
>>>>> callbacks and isn't otherwise synchronized with invocations of these
>>>>> callbacks, so maintain a count of in-progress calls, for evtchn_close()
>>>>> to wait to drop to zero before freeing the port (and dropping the lock).
>>>>
>>>> AFAICT, this callback is not the only place where the synchronization is
>>>> missing in the VM event code.
>>>>
>>>> For instance, vm_event_put_request() can also race against
>>>> vm_event_disable().
>>>>
>>>> So shouldn't we handle this issue properly in VM event?
>>>
>>> I suppose that's a question to the VM event folks rather than me?
>>
>> Yes. From my understanding of Tamas's e-mail, they are relying on the
>> monitoring software to do the right thing.
>>
>> I will refrain to comment on this approach. However, given the race is
>> much wider than the event channel, I would recommend to not add more
>> code in the event channel to deal with such problem.
>>
>> Instead, this should be fixed in the VM event code when someone has time
>> to harden the subsystem.
> 
> Are effectively saying I should now undo the addition of the
> refcounting, which was added in response to feedback from you?

Please point out where I made the request to use the refcounting...

I pointed out there was an issue with the VM event code. This was latter 
analysed as a wider issue. The VM event folks doesn't seem to be very 
concerned on the race, so I don't see the reason to try to fix it in the 
event channel code.

Cheers,

-- 
Julien Grall