Provide a proper invalidation method rather than relying on the netfs retiring
the cookie it has and getting a new one.  The problem with this is that isn't
easy for the netfs to make sure that it has completed/cancelled all its
outstanding storage and retrieval operations on the cookie it is retiring.

Instead, have the cache provide an invalidation method that will cancel or wait
for all currently outstanding operations before invalidating the cache, and
will cause new operations to queue up behind that.  Whilst invalidation is in
progress, some requests will be rejected until the cache can stack a barrier on
the operation queue to cause new operations to be deferred behind it.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Fix the state management of internal fscache operations and the accounting of
what operations are in what states.

This is done by:

 (1) Give struct fscache_operation a enum variable that directly represents the
     state it's currently in, rather than spreading this knowledge over a bunch
     of flags, who's processing the operation at the moment and whether it is
     queued or not.

     This makes it easier to write assertions to check the state at various
     points and to prevent invalid state transitions.

 (2) Add an 'operation complete' state and supply a function to indicate the
     completion of an operation (fscache_op_complete()) and make things call
     it.  The final call to fscache_put_operation() can then check that an op
     in the appropriate state (complete or cancelled).

 (3) Adjust the use of object->n_ops, ->n_in_progress, ->n_exclusive to better
     govern the state of an object:

	(a) The ->n_ops is now the number of extant operations on the object
	    and is now decremented by fscache_put_operation() only.

	(b) The ->n_in_progress is simply the number of objects that have been
	    taken off of the object's pending queue for the purposes of being
	    run.  This is decremented by fscache_op_complete() only.

	(c) The ->n_exclusive is the number of exclusive ops that have been
	    submitted and queued or are in progress.  It is decremented by
	    fscache_op_complete() and by fscache_cancel_op().

     fscache_put_operation() and fscache_operation_gc() now no longer try to
     clean up ->n_exclusive and ->n_in_progress.  That was leading to double
     decrements against fscache_cancel_op().

     fscache_cancel_op() now no longer decrements ->n_ops.  That was leading to
     double decrements against fscache_put_operation().

     fscache_submit_exclusive_op() now decides whether it has to queue an op
     based on ->n_in_progress being > 0 rather than ->n_ops > 0 as the latter
     will persist in being true even after all preceding operations have been
     cancelled or completed.  Furthermore, if an object is active and there are
     runnable ops against it, there must be at least one op running.

 (4) Add a remaining-pages counter (n_pages) to struct fscache_retrieval and
     provide a function to record completion of the pages as they complete.

     When n_pages reaches 0, the operation is deemed to be complete and
     fscache_op_complete() is called.

     Add calls to fscache_retrieval_complete() anywhere we've finished with a
     page we've been given to read or allocate for.  This includes places where
     we just return pages to the netfs for reading from the server and where
     accessing the cache fails and we discard the proposed netfs page.

The bugs in the unfixed state management manifest themselves as oopses like the
following where the operation completion gets out of sync with return of the
cookie by the netfs.  This is possible because the cache unlocks and returns
all the netfs pages before recording its completion - which means that there's
nothing to stop the netfs discarding them and returning the cookie.


FS-Cache: Cookie 'NFS.fh' still has outstanding reads
------------[ cut here ]------------
kernel BUG at fs/fscache/cookie.c:519!
invalid opcode: 0000 [#1] SMP
CPU 1
Modules linked in: cachefiles nfs fscache auth_rpcgss nfs_acl lockd sunrpc

Pid: 400, comm: kswapd0 Not tainted 3.1.0-rc7-fsdevel+ #1090                  /DG965RY
RIP: 0010:[<ffffffffa007050a>]  [<ffffffffa007050a>] __fscache_relinquish_cookie+0x170/0x343 [fscache]
RSP: 0018:ffff8800368cfb00  EFLAGS: 00010282
RAX: 000000000000003c RBX: ffff880023cc8790 RCX: 0000000000000000
RDX: 0000000000002f2e RSI: 0000000000000001 RDI: ffffffff813ab86c
RBP: ffff8800368cfb50 R08: 0000000000000002 R09: 0000000000000000
R10: ffff88003a1b7890 R11: ffff88001df6e488 R12: ffff880023d8ed98
R13: ffff880023cc8798 R14: 0000000000000004 R15: ffff88003b8bf370
FS:  0000000000000000(0000) GS:ffff88003bd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00000000008ba008 CR3: 0000000023d93000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process kswapd0 (pid: 400, threadinfo ffff8800368ce000, task ffff88003b8bf040)
Stack:
 ffff88003b8bf040 ffff88001df6e528 ffff88001df6e528 ffffffffa00b46b0
 ffff88003b8bf040 ffff88001df6e488 ffff88001df6e620 ffffffffa00b46b0
 ffff88001ebd04c8 0000000000000004 ffff8800368cfb70 ffffffffa00b2c91
Call Trace:
 [<ffffffffa00b2c91>] nfs_fscache_release_inode_cookie+0x3b/0x47 [nfs]
 [<ffffffffa008f25f>] nfs_clear_inode+0x3c/0x41 [nfs]
 [<ffffffffa0090df1>] nfs4_evict_inode+0x2f/0x33 [nfs]
 [<ffffffff810d8d47>] evict+0xa1/0x15c
 [<ffffffff810d8e2e>] dispose_list+0x2c/0x38
 [<ffffffff810d9ebd>] prune_icache_sb+0x28c/0x29b
 [<ffffffff810c56b7>] prune_super+0xd5/0x140
 [<ffffffff8109b615>] shrink_slab+0x102/0x1ab
 [<ffffffff8109d690>] balance_pgdat+0x2f2/0x595
 [<ffffffff8103e009>] ? process_timeout+0xb/0xb
 [<ffffffff8109dba3>] kswapd+0x270/0x289
 [<ffffffff8104c5ea>] ? __init_waitqueue_head+0x46/0x46
 [<ffffffff8109d933>] ? balance_pgdat+0x595/0x595
 [<ffffffff8104bf7a>] kthread+0x7f/0x87
 [<ffffffff813ad6b4>] kernel_thread_helper+0x4/0x10
 [<ffffffff81026b98>] ? finish_task_switch+0x45/0xc0
 [<ffffffff813abcdd>] ? retint_restore_args+0xe/0xe
 [<ffffffff8104befb>] ? __init_kthread_worker+0x53/0x53
 [<ffffffff813ad6b0>] ? gs_change+0xb/0xb
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Make fscache object state transition callbacks use workqueue instead
of slow-work.  New dedicated unbound CPU workqueue fscache_object_wq
is created.  get/put callbacks are renamed and modified to take
@object and called directly from the enqueue wrapper and the work
function.  While at it, make all open coded instances of get/put to
use fscache_get/put_object().

* Unbound workqueue is used.

* work_busy() output is printed instead of slow-work flags in object
  debugging outputs.  They mean basically the same thing bit-for-bit.

* sysctl fscache.object_max_active added to control concurrency.  The
  default value is nr_cpus clamped between 4 and
  WQ_UNBOUND_MAX_ACTIVE.

* slow_work_sleep_till_thread_needed() is replaced with fscache
  private implementation fscache_object_sleep_till_congested() which
  waits on fscache_object_wq congestion.

* debugfs support is dropped for now.  Tracing API based debug
  facility is planned to be added.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid Howells <dhowells@redhat.com>

CONFIG_SLOW_WORK_PROC was changed to CONFIG_SLOW_WORK_DEBUG, but not in all
instances.  Change the remaining instances.  This makes the debugfs file
display the time mark and the owner's description again.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Catch an overly long wait for an old, dying active object when we want to
replace it with a new one.  The probability is that all the slow-work threads
are hogged, and the delete can't get a look in.

What we do instead is:

 (1) if there's nothing in the slow work queue, we sleep until either the dying
     object has finished dying or there is something in the slow work queue
     behind which we can queue our object.

 (2) if there is something in the slow work queue, we return ETIMEDOUT to
     fscache_lookup_object(), which then puts us back on the slow work queue,
     presumably behind the deletion that we're blocked by.  We are then
     deferred for a while until we work our way back through the queue -
     without blocking a slow-work thread unnecessarily.

A backtrace similar to the following may appear in the log without this patch:

	INFO: task kslowd004:5711 blocked for more than 120 seconds.
	"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
	kslowd004     D 0000000000000000     0  5711      2 0x00000080
	 ffff88000340bb80 0000000000000046 ffff88002550d000 0000000000000000
	 ffff88002550d000 0000000000000007 ffff88000340bfd8 ffff88002550d2a8
	 000000000000ddf0 00000000000118c0 00000000000118c0 ffff88002550d2a8
	Call Trace:
	 [<ffffffff81058e21>] ? trace_hardirqs_on+0xd/0xf
	 [<ffffffffa011c4d8>] ? cachefiles_wait_bit+0x0/0xd [cachefiles]
	 [<ffffffffa011c4e1>] cachefiles_wait_bit+0x9/0xd [cachefiles]
	 [<ffffffff81353153>] __wait_on_bit+0x43/0x76
	 [<ffffffff8111ae39>] ? ext3_xattr_get+0x1ec/0x270
	 [<ffffffff813531ef>] out_of_line_wait_on_bit+0x69/0x74
	 [<ffffffffa011c4d8>] ? cachefiles_wait_bit+0x0/0xd [cachefiles]
	 [<ffffffff8104c125>] ? wake_bit_function+0x0/0x2e
	 [<ffffffffa011bc79>] cachefiles_mark_object_active+0x203/0x23b [cachefiles]
	 [<ffffffffa011c209>] cachefiles_walk_to_object+0x558/0x827 [cachefiles]
	 [<ffffffffa011a429>] cachefiles_lookup_object+0xac/0x12a [cachefiles]
	 [<ffffffffa00aa1e9>] fscache_lookup_object+0x1c7/0x214 [fscache]
	 [<ffffffffa00aafc5>] fscache_object_state_machine+0xa5/0x52d [fscache]
	 [<ffffffffa00ab4ac>] fscache_object_slow_work_execute+0x5f/0xa0 [fscache]
	 [<ffffffff81082093>] slow_work_execute+0x18f/0x2d1
	 [<ffffffff8108239a>] slow_work_thread+0x1c5/0x308
	 [<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34
	 [<ffffffff810821d5>] ? slow_work_thread+0x0/0x308
	 [<ffffffff8104be91>] kthread+0x7a/0x82
	 [<ffffffff8100beda>] child_rip+0xa/0x20
	 [<ffffffff8100b87c>] ? restore_args+0x0/0x30
	 [<ffffffff8104be17>] ? kthread+0x0/0x82
	 [<ffffffff8100bed0>] ? child_rip+0x0/0x20
	1 lock held by kslowd004/5711:
	 #0:  (&sb->s_type->i_mutex_key#7/1){+.+.+.}, at: [<ffffffffa011be64>] cachefiles_walk_to_object+0x1b3/0x827 [cachefiles]
Signed-off-by: NDavid Howells <dhowells@redhat.com>

FS-Cache objects have an FSCACHE_OBJECT_EV_REQUEUE event that can theoretically
be raised to ask the state machine to requeue the object for further processing
before the work function returns to the slow-work facility.

However, fscache_object_work_execute() was clearing that bit before checking
the event mask to see whether the object has any pending events that require it
to be requeued immediately.

Instead, the bit should be cleared after the check and enqueue.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Start processing an object's operations when that object moves into the DYING
state as the object cannot be destroyed until all its outstanding operations
have completed.

Furthermore, make sure that read and allocation operations handle being woken
up on a dead object.  Such events are recorded in the Allocs.abt and
Retrvls.abt statistics as viewable through /proc/fs/fscache/stats.

The code for waiting for object activation for the read and allocation
operations is also extracted into its own function as it is much the same in
all cases, differing only in the stats incremented.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

We must make sure that FSCACHE_COOKIE_LOOKING_UP is cleared on lookup failure
(if an object reaches the LC_DYING state), and we should clear it before
clearing FSCACHE_COOKIE_CREATING.

If this doesn't happen then fscache_wait_for_deferred_lookup() may hold
allocation and retrieval operations indefinitely until they're interrupted by
signals - which in turn pins the dying object until they go away.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

The object-available state in the object processing state machine (as
processed by fscache_object_available()) can't rely on the cookie to be
available because the FSCACHE_COOKIE_CREATING bit may have been cleared by
fscache_obtained_object() prior to the object being put into the
FSCACHE_OBJECT_AVAILABLE state.

Clearing the FSCACHE_COOKIE_CREATING bit on a cookie permits
__fscache_relinquish_cookie() to proceed and detach the cookie from the
object.

To deal with this, we don't dereference object->cookie in
fscache_object_available() if the object has already been detached.

In addition, a couple of assertions are added into fscache_drop_object() to
make sure the object is unbound from the cookie before it gets there.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Count entries to and exits from cache operation table functions.  Maintain
these as a single counter that's added to or removed from as appropriate.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Allow the current state of all fscache objects to be dumped by doing:

	cat /proc/fs/fscache/objects

By default, all objects and all fields will be shown.  This can be restricted
by adding a suitable key to one of the caller's keyrings (such as the session
keyring):

	keyctl add user fscache:objlist "<restrictions>" @s

The <restrictions> are:

	K	Show hexdump of object key (don't show if not given)
	A	Show hexdump of object aux data (don't show if not given)

And paired restrictions:

	C	Show objects that have a cookie
	c	Show objects that don't have a cookie
	B	Show objects that are busy
	b	Show objects that aren't busy
	W	Show objects that have pending writes
	w	Show objects that don't have pending writes
	R	Show objects that have outstanding reads
	r	Show objects that don't have outstanding reads
	S	Show objects that have slow work queued
	s	Show objects that don't have slow work queued

If neither side of a restriction pair is given, then both are implied.  For
example:

	keyctl add user fscache:objlist KB @s

shows objects that are busy, and lists their object keys, but does not dump
their auxiliary data.  It also implies "CcWwRrSs", but as 'B' is given, 'b' is
not implied.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Annotate slow-work runqueue proc lines for FS-Cache work items.  Objects
include the object ID and the state.  Operations include the object ID, the
operation ID and the operation type and state.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Wait for outstanding slow work items belonging to a module to clear when
unregistering that module as a user of the facility.  This prevents the put_ref
code of a work item from being taken away before it returns.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Implement the cache object management state machine.

The following documentation is added to illuminate the working of this state
machine.  It will also be added as:

	Documentation/filesystems/caching/object.txt

	     ====================================================
	     IN-KERNEL CACHE OBJECT REPRESENTATION AND MANAGEMENT
	     ====================================================

==============
REPRESENTATION
==============

FS-Cache maintains an in-kernel representation of each object that a netfs is
currently interested in.  Such objects are represented by the fscache_cookie
struct and are referred to as cookies.

FS-Cache also maintains a separate in-kernel representation of the objects that
a cache backend is currently actively caching.  Such objects are represented by
the fscache_object struct.  The cache backends allocate these upon request, and
are expected to embed them in their own representations.  These are referred to
as objects.

There is a 1:N relationship between cookies and objects.  A cookie may be
represented by multiple objects - an index may exist in more than one cache -
or even by no objects (it may not be cached).

Furthermore, both cookies and objects are hierarchical.  The two hierarchies
correspond, but the cookies tree is a superset of the union of the object trees
of multiple caches:

	    NETFS INDEX TREE               :      CACHE 1     :      CACHE 2
	                                   :                  :
	                                   :   +-----------+  :
	                          +----------->|  IObject  |  :
	      +-----------+       |        :   +-----------+  :
	      |  ICookie  |-------+        :         |        :
	      +-----------+       |        :         |        :   +-----------+
	            |             +------------------------------>|  IObject  |
	            |                      :         |        :   +-----------+
	            |                      :         V        :         |
	            |                      :   +-----------+  :         |
	            V             +----------->|  IObject  |  :         |
	      +-----------+       |        :   +-----------+  :         |
	      |  ICookie  |-------+        :         |        :         V
	      +-----------+       |        :         |        :   +-----------+
	            |             +------------------------------>|  IObject  |
	      +-----+-----+                :         |        :   +-----------+
	      |           |                :         |        :         |
	      V           |                :         V        :         |
	+-----------+     |                :   +-----------+  :         |
	|  ICookie  |------------------------->|  IObject  |  :         |
	+-----------+     |                :   +-----------+  :         |
	      |           V                :         |        :         V
	      |     +-----------+          :         |        :   +-----------+
	      |     |  ICookie  |-------------------------------->|  IObject  |
	      |     +-----------+          :         |        :   +-----------+
	      V           |                :         V        :         |
	+-----------+     |                :   +-----------+  :         |
	|  DCookie  |------------------------->|  DObject  |  :         |
	+-----------+     |                :   +-----------+  :         |
	                  |                :                  :         |
	          +-------+-------+        :                  :         |
	          |               |        :                  :         |
	          V               V        :                  :         V
	    +-----------+   +-----------+  :                  :   +-----------+
	    |  DCookie  |   |  DCookie  |------------------------>|  DObject  |
	    +-----------+   +-----------+  :                  :   +-----------+
	                                   :                  :

In the above illustration, ICookie and IObject represent indices and DCookie
and DObject represent data storage objects.  Indices may have representation in
multiple caches, but currently, non-index objects may not.  Objects of any type
may also be entirely unrepresented.

As far as the netfs API goes, the netfs is only actually permitted to see
pointers to the cookies.  The cookies themselves and any objects attached to
those cookies are hidden from it.

===============================
OBJECT MANAGEMENT STATE MACHINE
===============================

Within FS-Cache, each active object is managed by its own individual state
machine.  The state for an object is kept in the fscache_object struct, in
object->state.  A cookie may point to a set of objects that are in different
states.

Each state has an action associated with it that is invoked when the machine
wakes up in that state.  There are four logical sets of states:

 (1) Preparation: states that wait for the parent objects to become ready.  The
     representations are hierarchical, and it is expected that an object must
     be created or accessed with respect to its parent object.

 (2) Initialisation: states that perform lookups in the cache and validate
     what's found and that create on disk any missing metadata.

 (3) Normal running: states that allow netfs operations on objects to proceed
     and that update the state of objects.

 (4) Termination: states that detach objects from their netfs cookies, that
     delete objects from disk, that handle disk and system errors and that free
     up in-memory resources.

In most cases, transitioning between states is in response to signalled events.
When a state has finished processing, it will usually set the mask of events in
which it is interested (object->event_mask) and relinquish the worker thread.
Then when an event is raised (by calling fscache_raise_event()), if the event
is not masked, the object will be queued for processing (by calling
fscache_enqueue_object()).

PROVISION OF CPU TIME
---------------------

The work to be done by the various states is given CPU time by the threads of
the slow work facility (see Documentation/slow-work.txt).  This is used in
preference to the workqueue facility because:

 (1) Threads may be completely occupied for very long periods of time by a
     particular work item.  These state actions may be doing sequences of
     synchronous, journalled disk accesses (lookup, mkdir, create, setxattr,
     getxattr, truncate, unlink, rmdir, rename).

 (2) Threads may do little actual work, but may rather spend a lot of time
     sleeping on I/O.  This means that single-threaded and 1-per-CPU-threaded
     workqueues don't necessarily have the right numbers of threads.

LOCKING SIMPLIFICATION
----------------------

Because only one worker thread may be operating on any particular object's
state machine at once, this simplifies the locking, particularly with respect
to disconnecting the netfs's representation of a cache object (fscache_cookie)
from the cache backend's representation (fscache_object) - which may be
requested from either end.

=================
THE SET OF STATES
=================

The object state machine has a set of states that it can be in.  There are
preparation states in which the object sets itself up and waits for its parent
object to transit to a state that allows access to its children:

 (1) State FSCACHE_OBJECT_INIT.

     Initialise the object and wait for the parent object to become active.  In
     the cache, it is expected that it will not be possible to look an object
     up from the parent object, until that parent object itself has been looked
     up.

There are initialisation states in which the object sets itself up and accesses
disk for the object metadata:

 (2) State FSCACHE_OBJECT_LOOKING_UP.

     Look up the object on disk, using the parent as a starting point.
     FS-Cache expects the cache backend to probe the cache to see whether this
     object is represented there, and if it is, to see if it's valid (coherency
     management).

     The cache should call fscache_object_lookup_negative() to indicate lookup
     failure for whatever reason, and should call fscache_obtained_object() to
     indicate success.

     At the completion of lookup, FS-Cache will let the netfs go ahead with
     read operations, no matter whether the file is yet cached.  If not yet
     cached, read operations will be immediately rejected with ENODATA until
     the first known page is uncached - as to that point there can be no data
     to be read out of the cache for that file that isn't currently also held
     in the pagecache.

 (3) State FSCACHE_OBJECT_CREATING.

     Create an object on disk, using the parent as a starting point.  This
     happens if the lookup failed to find the object, or if the object's
     coherency data indicated what's on disk is out of date.  In this state,
     FS-Cache expects the cache to create

     The cache should call fscache_obtained_object() if creation completes
     successfully, fscache_object_lookup_negative() otherwise.

     At the completion of creation, FS-Cache will start processing write
     operations the netfs has queued for an object.  If creation failed, the
     write ops will be transparently discarded, and nothing recorded in the
     cache.

There are some normal running states in which the object spends its time
servicing netfs requests:

 (4) State FSCACHE_OBJECT_AVAILABLE.

     A transient state in which pending operations are started, child objects
     are permitted to advance from FSCACHE_OBJECT_INIT state, and temporary
     lookup data is freed.

 (5) State FSCACHE_OBJECT_ACTIVE.

     The normal running state.  In this state, requests the netfs makes will be
     passed on to the cache.

 (6) State FSCACHE_OBJECT_UPDATING.

     The state machine comes here to update the object in the cache from the
     netfs's records.  This involves updating the auxiliary data that is used
     to maintain coherency.

And there are terminal states in which an object cleans itself up, deallocates
memory and potentially deletes stuff from disk:

 (7) State FSCACHE_OBJECT_LC_DYING.

     The object comes here if it is dying because of a lookup or creation
     error.  This would be due to a disk error or system error of some sort.
     Temporary data is cleaned up, and the parent is released.

 (8) State FSCACHE_OBJECT_DYING.

     The object comes here if it is dying due to an error, because its parent
     cookie has been relinquished by the netfs or because the cache is being
     withdrawn.

     Any child objects waiting on this one are given CPU time so that they too
     can destroy themselves.  This object waits for all its children to go away
     before advancing to the next state.

 (9) State FSCACHE_OBJECT_ABORT_INIT.

     The object comes to this state if it was waiting on its parent in
     FSCACHE_OBJECT_INIT, but its parent died.  The object will destroy itself
     so that the parent may proceed from the FSCACHE_OBJECT_DYING state.

(10) State FSCACHE_OBJECT_RELEASING.
(11) State FSCACHE_OBJECT_RECYCLING.

     The object comes to one of these two states when dying once it is rid of
     all its children, if it is dying because the netfs relinquished its
     cookie.  In the first state, the cached data is expected to persist, and
     in the second it will be deleted.

(12) State FSCACHE_OBJECT_WITHDRAWING.

     The object transits to this state if the cache decides it wants to
     withdraw the object from service, perhaps to make space, but also due to
     error or just because the whole cache is being withdrawn.

(13) State FSCACHE_OBJECT_DEAD.

     The object transits to this state when the in-memory object record is
     ready to be deleted.  The object processor shouldn't ever see an object in
     this state.

THE SET OF EVENTS
-----------------

There are a number of events that can be raised to an object state machine:

 (*) FSCACHE_OBJECT_EV_UPDATE

     The netfs requested that an object be updated.  The state machine will ask
     the cache backend to update the object, and the cache backend will ask the
     netfs for details of the change through its cookie definition ops.

 (*) FSCACHE_OBJECT_EV_CLEARED

     This is signalled in two circumstances:

     (a) when an object's last child object is dropped and

     (b) when the last operation outstanding on an object is completed.

     This is used to proceed from the dying state.

 (*) FSCACHE_OBJECT_EV_ERROR

     This is signalled when an I/O error occurs during the processing of some
     object.

 (*) FSCACHE_OBJECT_EV_RELEASE
 (*) FSCACHE_OBJECT_EV_RETIRE

     These are signalled when the netfs relinquishes a cookie it was using.
     The event selected depends on whether the netfs asks for the backing
     object to be retired (deleted) or retained.

 (*) FSCACHE_OBJECT_EV_WITHDRAW

     This is signalled when the cache backend wants to withdraw an object.
     This means that the object will have to be detached from the netfs's
     cookie.

Because the withdrawing releasing/retiring events are all handled by the object
state machine, it doesn't matter if there's a collision with both ends trying
to sever the connection at the same time.  The state machine can just pick
which one it wants to honour, and that effects the other.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NSteve Dickson <steved@redhat.com>
Acked-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: NAl Viro <viro@zeniv.linux.org.uk>
Tested-by: NDaire Byrne <Daire.Byrne@framestore.com>