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

[Christian Borntraeger]

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 <address@hidden>
>>>> AuthorDate: Fri Feb 20 17:26:52 2015 +0100
>>>> Commit:     Kevin Wolf <address@hidden>
>>>> 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?
>>>
>>> Christian, I can't reproduce this on my x86 box with virtio-blk-pci. Do you
>>> have a reproducer for x86? Or could you collect backtraces for all the 
>>> threads
>>> in QEMU when it hangs?
>>>
>>> My long shot is that the main loop is blocked at aio_context_acquire(ctx),
>>> while the iothread of that ctx is blocked at aio_poll(ctx, blocking).
>>
>> Here is a backtrace on s390. I need 2 or more disks, (one is not enough).
> 
> It shows iothreads and main loop are all waiting for events, and the vcpu
> threads are running guest code.
> 
> It could be the requests being leaked. Do you see this problem with a regular
> file based image or null-co driver? Maybe we're missing something about the
> AioContext in block/null.c.

It seems to run with normal file based images. As soon as I have two or more 
null-aio
devices it hangs pretty soon when doing a reboot loop.

Christian