Re: [Qemu-devel] "iothread: release iothread around aio_poll" causes random hangs at startup

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 10.06.2015 um 11:34 schrieb Fam Zheng:
> On Wed, 06/10 11:18, Christian Borntraeger wrote:
>> Am 10.06.2015 um 04:12 schrieb Fam Zheng:
>>> On Tue, 06/09 11:01, Christian Borntraeger wrote:
>>>> Am 09.06.2015 um 04:28 schrieb Fam Zheng:
>>>>> On Tue, 06/02 16:36, Christian Borntraeger wrote:
>>>>>> Paolo,
>>>>>>
>>>>>> I bisected 
>>>>>> commit a0710f7995f914e3044e5899bd8ff6c43c62f916
>>>>>> Author:     Paolo Bonzini <pbonzini@redhat.com>
>>>>>> AuthorDate: Fri Feb 20 17:26:52 2015 +0100
>>>>>> Commit:     Kevin Wolf <kwolf@redhat.com>
>>>>>> CommitDate: Tue Apr 28 15:36:08 2015 +0200
>>>>>>
>>>>>>     iothread: release iothread around aio_poll
>>>>>>
>>>>>> to cause a problem with hanging guests.
>>>>>>
>>>>>> Having many guests all with a kernel/ramdisk (via -kernel) and
>>>>>> several null block devices will result in hangs. All hanging 
>>>>>> guests are in partition detection code waiting for an I/O to return
>>>>>> so very early maybe even the first I/O.
>>>>>>
>>>>>> Reverting that commit "fixes" the hangs.
>>>>>> Any ideas?


For what its worth, I can no longer reproduce the issue on
current master + cherry-pick of a0710f7995f (iothread: release iothread around aio_poll)

bisect tells me that

commit 53ec73e264f481b79b52efcadc9ceb8f8996975c
Author:     Fam Zheng <famz@redhat.com>
AuthorDate: Fri May 29 18:53:14 2015 +0800
Commit:     Stefan Hajnoczi <stefanha@redhat.com>
CommitDate: Tue Jul 7 14:27:14 2015 +0100

    block: Use bdrv_drain to replace uncessary bdrv_drain_all

made the problem will blk-null go away. I still dont understand why.

Christian