Re: [Qemu-devel] [RFC PATCH 0/2] establish nesting rule of BQL vs cpu-exclusive

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/08/19 14:47, Roman Kagan wrote:
> On Fri, Jun 21, 2019 at 12:49:07PM +0000, Roman Kagan wrote:
>> On Thu, Jun 06, 2019 at 01:22:33PM +0000, Roman Kagan wrote:
>>> On Mon, May 27, 2019 at 11:05:38AM +0000, Roman Kagan wrote:
>>>> On Thu, May 23, 2019 at 12:31:16PM +0100, Alex Bennée wrote:
>>>>>
>>>>> Roman Kagan <rkagan@virtuozzo.com> writes:
>>>>>
>>>>>> I came across the following AB-BA deadlock:
>>>>>>
>>>>>>     vCPU thread                             main thread
>>>>>>     -----------                             -----------
>>>>>> async_safe_run_on_cpu(self,
>>>>>>                       async_synic_update)
>>>>>> ...                                         [cpu hot-add]
>>>>>> process_queued_cpu_work()
>>>>>>   qemu_mutex_unlock_iothread()
>>>>>>                                             [grab BQL]
>>>>>>   start_exclusive()                         cpu_list_add()
>>>>>>   async_synic_update()                        finish_safe_work()
>>>>>>     qemu_mutex_lock_iothread()                  cpu_exec_start()
>>>>>>
>>>>>> ATM async_synic_update seems to be the only async safe work item that
>>>>>> grabs BQL.  However it isn't quite obvious that it shouldn't; in the
>>>>>> past there were more examples of this (e.g.
>>>>>> memory_region_do_invalidate_mmio_ptr).
>>>>>>
>>>>>> It looks like the problem is generally in the lack of the nesting rule
>>>>>> for cpu-exclusive sections against BQL, so I thought I would try to
>>>>>> address that.  This patchset is my feeble attempt at this; I'm not sure
>>>>>> I fully comprehend all the consequences (rather, I'm sure I don't) hence
>>>>>> RFC.
>>>>>
>>>>> Hmm I think this is an area touched by:
>>>>>
>>>>>   Subject: [PATCH v7 00/73] per-CPU locks
>>>>>   Date: Mon,  4 Mar 2019 13:17:00 -0500
>>>>>   Message-Id: <20190304181813.8075-1-cota@braap.org>
>>>>>
>>>>> which has stalled on it's path into the tree. Last time I checked it
>>>>> explicitly handled the concept of work that needed the BQL and work that
>>>>> didn't.
>>>>
>>>> I'm still trying to get my head around that patchset, but it looks like
>>>> it changes nothing in regards to cpu-exclusive sections and safe work,
>>>> so it doesn't make the problem go.
>>>>
>>>>> How do you trigger your deadlock? Just hot-pluging CPUs?
>>>>
>>>> Yes.  The window is pretty narrow so I only saw it once although this
>>>> test (where the vms are started and stopped and the cpus are plugged in
>>>> and out) is in our test loop for quite a bit (probably 2+ years).
>>>>
>>>> Roman.
>>>
>>> ping?
>>
>> ping?
> 
> ping?
> 

Queued for 4.2.

Paolo