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apport-collect 1776254

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This change has been made by an automated script, maintained by the
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** Changed in: linux (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1776254

Title:
  CacheFiles: Error: Overlong wait for old active object to go away.

Status in linux package in Ubuntu:
  Incomplete

Bug description:
  == SRU Justification ==

  [Impact]
  Oops during heavy NFS + FSCache + Cachefiles use:

   CacheFiles: Error: Overlong wait for old active object to go away.
   BUG: unable to handle kernel NULL pointer dereference at 0000000000000002

   CacheFiles: Error: Object already active
   kernel BUG at fs/cachefiles/namei.c:163!

  [Cause]
    In a heavily loaded system with big files being read and truncated,
    an fscache object for a cookie is being dropped and a new object being 
looked.
    The new object being looked for has to wait for the old object to go away 
before the
    new object is moved to active state.

  [Fix]
       Clear the flag 'CACHEFILES_OBJECT_ACTIVE' for the new object when 
retrying 
       the object lookup.
       Remove the BUG() for the case where the old object is still being dropped
       and convert to WARN()

  [Testcase]
  A user has run ~100 hours of NFS stress tests and not seen this bug recur.

  [Regression Potential]
   - Limited to fscache/cachefiles.

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