Re: [Qemu-devel] thread-pool.c race condition?

[John Snow <jsnow@redhat.com>]

On 04/02/2015 12:43 PM, Paolo Bonzini wrote:
>
>
> On 02/04/2015 18:26, Stefan Hajnoczi wrote:
>> John Snow has reported that qemu-io can hang when the host is under
>> heavy load.  He made the following observations in gdb:
>>
>> 1. The program is sitting in aio_poll() (called by bdrv_prwv_co())
>> waiting for request completion.
>>
>> 2. The thread pool has a ThreadPoolElement with ->state == THREAD_DONE.
>>
>> The ThreadPoolElement should have been reaped by
>> thread_pool_completion_bh() and its callback invoked.  For some reason
>> this didn't happen and the program is blocked in poll(2) waiting.
>>
>> This suggests a race condition in thread-pool.c or qemu_bh_schedule()
>> (used to complete ThreadPoolElement from a QEMU event loop).
>>
>> I don't have a good theory why this happens yet.  Just wanted to share
>> in case someone else hits this problem.
>
> Laszlo hit something very similar fairly easily with virtio-scsi (but
> not virtio-blk!) on aarch64 hosts.  Any attempt to debug it (ranging
> from compilation with -O0 to tracing) made it disappear.  A reliable
> reproducer with qemu-io would be a dream...
>
> Paolo
>

Unfortunately for you, I hit it by running qemu-iotests on my laptop 
overnight and I suspect it's triggered by my screensavers hogging CPU 
when I am AFK...

I hit it pretty reliably (100% of the time I tried to run tests while 
AFK -- three independent screensavers running on three monitors) two 
weeks ago, but haven't seen it recently.

I'll keep you posted...