title: What is a fence-hang / handling fence / blown-fence error?
body: <p>I had an iOS iPad app crash and got the following crash log from the Devices tab:</p>
  
  <pre><code>Date/Time:       2017-10-13 15:06:22 -0600
  OS Version:      iPhone OS 11.1 (Build 15B5066f)
  Architecture:    arm64
  Report Version:  26
  
  Data Source:     KPerf Lightweight PET
  Kernel Cache:    0xffffffe000000000 1C4A9D92-74B9-1F20-096E-27915D7A6C03
  Reason:          Fence-hang-Vocab: 505ms handling fence, blown-fence (fence duration=505ms)
  
  Command:         Vocab
  Path:            /private/var/containers/Bundle/Application/9F4A7C15-D23E-46C8-B724-84C57D83B455/Vocab.app/Vocab
  Identifier:      com.mycompany.Vocab
  Version:         1.4.0 (420)
  Parent:          debugserver [1467]
  PID:             1469
  
  Duration:        0.55s
  Steps:           11 (50ms sampling interval)
  
  Hardware model:  J71AP
  Active cpus:     2
  </code></pre>
  
  <p>Any insight into what this error is and what causes it?</p>