[Bug 1457639] [NEW] qemu-img qcow2 conversion hangs on large core systems

[dann frazier]

Public bug reported:

[Impact]
qemu-img frequently hangs when converting a file to qcow2 on a system with a large number of cores.

[Test Case]
On a system with a large number of cores (my test system has 48), run:

wget http://cloud-images.ubuntu.com/trusty/current/trusty-server-cloudimg-arm64-disk1.img -O disk.img
for i in $(seq 1 10); do qemu-img convert -O qcow2 disk.img disk.qcow2; done

Mine always hangs within the first 3 iterations.

[Regression Risk]
Fix is a clean cherry pick from upstream, which should give us a clear path to resolution if regressions are found.

** Affects: qemu (Ubuntu)
     Importance: High
         Status: Confirmed

** Affects: qemu (Ubuntu Vivid)
     Importance: High
         Status: Confirmed

** Affects: qemu (Ubuntu Wily)
     Importance: High
         Status: Confirmed


** Tags: hs-arm64

** Also affects: qemu (Ubuntu Wily)
   Importance: High
       Status: Confirmed

** Also affects: qemu (Ubuntu Vivid)
   Importance: Undecided
       Status: New

** Changed in: qemu (Ubuntu Vivid)
       Status: New => Confirmed

** Changed in: qemu (Ubuntu Vivid)
   Importance: Undecided => High

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https://bugs.launchpad.net/bugs/1457639

Title:
  qemu-img qcow2 conversion hangs on large core systems

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