When an object is being marked as no longer live, do this under the object
spinlock to prevent a race with operation submission targeted on that object.

The problem occurs due to the following pair of intertwined sequences when the
cache tries to create an object that would take it over the hard available
space limit:

 NETFS INTERFACE
 ===============
 (A) The netfs calls fscache_acquire_cookie().  object creation is deferred to
     the object state machine and the netfs is allowed to continue.

	OBJECT STATE MACHINE KTHREAD
	============================
	(1) The object is looked up on disk by fscache_look_up_object()
	    calling cachefiles_walk_to_object().  The latter finds that the
	    object is not yet represented on disk and calls
	    fscache_object_lookup_negative().

	(2) fscache_object_lookup_negative() sets FSCACHE_COOKIE_NO_DATA_YET
	    and clears FSCACHE_COOKIE_LOOKING_UP, thus allowing the netfs to
	    start queuing read operations.

 (B) The netfs calls fscache_read_or_alloc_pages().  This calls
     fscache_wait_for_deferred_lookup() which sees FSCACHE_COOKIE_LOOKING_UP
     become clear, allowing the read to begin.

 (C) A read operation is set up and passed to fscache_submit_op() to deal
     with.

	(3) cachefiles_walk_to_object() calls cachefiles_has_space(), which
	    fails (or one of the file operations to create stuff fails).
	    cachefiles returns an error to fscache.

	(4) fscache_look_up_object() transits to the LOOKUP_FAILURE state,

	(5) fscache_lookup_failure() sets FSCACHE_OBJECT_LOOKED_UP and
	    FSCACHE_COOKIE_UNAVAILABLE and clears FSCACHE_COOKIE_LOOKING_UP
	    then transits to the KILL_OBJECT state.

	(6) fscache_kill_object() clears FSCACHE_OBJECT_IS_LIVE in an attempt
	    to reject any further requests from the netfs.

	(7) object->n_ops is examined and found to be 0.
	    fscache_kill_object() transits to the DROP_OBJECT state.

 (D) fscache_submit_op() locks the object spinlock, sees if it can dispatch
     the op immediately by calling fscache_object_is_active() - which fails
     since FSCACHE_OBJECT_IS_AVAILABLE has not yet been set.

 (E) fscache_submit_op() then tests FSCACHE_OBJECT_LOOKED_UP - which is set.
     It then queues the object and increments object->n_ops.

	(8) fscache_drop_object() releases the object and eventually
	    fscache_put_object() calls cachefiles_put_object() which suffers
	    an assertion failure here:

		ASSERTCMP(object->fscache.n_ops, ==, 0);

Locking the object spinlock in step (6) around the clearance of
FSCACHE_OBJECT_IS_LIVE ensures that the the decision trees in
fscache_submit_op() and fscache_submit_exclusive_op() don't see the IS_LIVE
flag being cleared mid-decision: either the op is queued before step (7) - in
which case fscache_kill_object() will see n_ops>0 and will deal with the op -
or the op will be rejected.

This, combined with rejecting op submission if the target object is dying, fix
the problem.

The problem shows up as the following oops:

CacheFiles: Assertion failed
CacheFiles: 1 == 0 is false
------------[ cut here ]------------
kernel BUG at ../fs/cachefiles/interface.c:339!
...
RIP: 0010:[<ffffffffa014fd9c>]  [<ffffffffa014fd9c>] cachefiles_put_object+0x2a4/0x301 [cachefiles]
...
Call Trace:
 [<ffffffffa008674b>] fscache_put_object+0x18/0x21 [fscache]
 [<ffffffffa00883e6>] fscache_object_work_func+0x3ba/0x3c9 [fscache]
 [<ffffffff81054dad>] process_one_work+0x226/0x441
 [<ffffffff81055d91>] worker_thread+0x273/0x36b
 [<ffffffff81055b1e>] ? rescuer_thread+0x2e1/0x2e1
 [<ffffffff81059b9d>] kthread+0x10e/0x116
 [<ffffffff81059a8f>] ? kthread_create_on_node+0x1bb/0x1bb
 [<ffffffff815579ac>] ret_from_fork+0x7c/0xb0
 [<ffffffff81059a8f>] ? kthread_create_on_node+0x1bb/0x1bb
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Reviewed-by: NSteve Dickson <steved@redhat.com>
Acked-by: NJeff Layton <jeff.layton@primarydata.com>