I have confirmation from a user running NFS stress tests on Xenial with
this kernel and has not seen any issues. Changed tag to verification-

** Tags removed: verification-needed-xenial
** Tags added: verification-done-xenial

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  FS-Cache: Assertion failed: FS-Cache: 6 == 5 is false

Status in linux package in Ubuntu:
Status in linux source package in Trusty:
  Fix Committed
Status in linux source package in Xenial:
  Fix Committed
Status in linux source package in Artful:
  Fix Committed
Status in linux source package in Bionic:
  Fix Committed

Bug description:
  == SRU Justification ==

  Oops during heavy NFS + FSCache use:

  [81738.886634] FS-Cache: 
  [81738.888281] FS-Cache: Assertion failed
  [81738.889461] FS-Cache: 6 == 5 is false
  [81738.890625] ------------[ cut here ]------------
  [81738.891706] kernel BUG at 

  6 == 5 represents an operation being DEAD when it was not expected to

  There is a race in fscache and cachefiles. 

  One thread is in cachefiles_read_waiter:
   1) object->work_lock is taken.
   2) the operation is added to the to_do list.
   3) the work lock is dropped.
   4) fscache_enqueue_retrieval is called, which takes a reference.

  Another thread is in cachefiles_read_copier:
   1) object->work_lock is taken
   2) an item is popped off the to_do list.
   3) object->work_lock is dropped.
   4) some processing is done on the item, and fscache_put_retrieval() is 
called, dropping a reference.

  Now if the this process in cachefiles_read_copier takes place
  *between* steps 3 and 4 in cachefiles_read_waiter, a reference will be
  dropped before it is taken, which leads to the objects reference count
  hitting zero, which leads to lifecycle events for the object happening
  too soon, leading to the assertion failure later on.

  (This is simplified and clarified from the original upstream analysis
  for this patch at https://www.redhat.com/archives/linux-
  cachefs/2018-February/msg00001.html and from a similar patch with a
  different approach to fixing the bug at

  Move fscache_enqueue_retrieval under the lock in cachefiles_read_waiter. This 
means that the object cannot be popped off the to_do list until it is in a 
fully consistent state with the reference taken.

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

  [Regression Potential]
   - Limited to fscache/cachefiles. 
   - The change makes things more conservative (doing more under lock) so 
that's reassuring. 
   - There may be performance impacts but none have been observed so far.

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