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Date: Tue, 02 Jun 2015 16:36:55 +0200
From: Christian Borntraeger <borntraeger@de.ibm.com>
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Subject: [Qemu-devel] "iothread: release iothread around aio_poll" causes
 random hangs at startup
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>

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?

Christian

PS: A guest xml looks like


<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>test2</name>
  <uuid>13bd8253-9abb-4be8-9399-73b899762aaa</uuid>
  <memory unit='KiB'>286720</memory>
  <currentMemory unit='KiB'>286720</currentMemory>
  <vcpu placement='static'>10</vcpu>
  <iothreads>8</iothreads>
  <cputune>
    <shares>12</shares>
  </cputune>
  <os>
    <type arch='s390x' machine='s390-ccw-virtio'>hvm</type>
    <kernel>/boot/vmlinux-4.0.0+</kernel>
    <initrd>/boot/ramdisk.reboot</initrd>
    <cmdline>root=/dev/ram0</cmdline>
    <boot dev='hd'/>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>preserve</on_crash>
  <devices>
    <controller type='usb' index='0' model='none'/>
    <console type='pty'>
      <target type='sclp' port='0'/>
    </console>
    <memballoon model='none'/>
  </devices>
  <qemu:commandline>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null1,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null1,serial=null1,iothread=iothread1'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null2,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null2,serial=null2,iothread=iothread2'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null3,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null3,serial=null3,iothread=iothread3'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null4,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null4,serial=null4,iothread=iothread4'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null5,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null5,serial=null5,iothread=iothread5'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null6,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null6,serial=null6,iothread=iothread6'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null7,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null7,serial=null7,iothread=iothread7'/>
    <qemu:arg value='-drive'/>
    <qemu:arg value='driver=null-aio,id=null8,if=none,size=100G'/>
    <qemu:arg value='-device'/>
    <qemu:arg value='virtio-blk-ccw,drive=null8,serial=null8,iothread=iothread8'/>
  </qemu:commandline>
</domain>