Merge tag 'fscache-fixes-20180725' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs

Pull fscache/cachefiles fixes from David Howells:

 - Allow cancelled operations to be queued so they can be cleaned up.

 - Fix a refcounting bug in the monitoring of reads on backend files
   whereby a race can occur between monitor objects being listed for
   work, the work processing being queued and the work processor running
   and destroying the monitor objects.

 - Fix a ref overput in object attachment, whereby a tentatively
   considered object is put in error handling without first being 'got'.

 - Fix a missing clear of the CACHEFILES_OBJECT_ACTIVE flag whereby an
   assertion occurs when we retry because it seems the object is now
   active.

 - Wait rather BUG'ing on an object collision in the depths of
   cachefiles as the active object should be being cleaned up - also
   depends on the one above.

* tag 'fscache-fixes-20180725' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs:
  cachefiles: Wait rather than BUG'ing on "Unexpected object collision"
  cachefiles: Fix missing clear of the CACHEFILES_OBJECT_ACTIVE flag
  fscache: Fix reference overput in fscache_attach_object() error handling
  cachefiles: Fix refcounting bug in backing-file read monitoring
  fscache: Allow cancelled operations to be enqueued

1 files changed, 12 insertions, 5 deletions

diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 5082c8a49686..40f7595aad10 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -27,6 +27,7 @@ static int cachefiles_read_waiter(wait_queue_entry_t *wait, unsigned mode,
 	struct cachefiles_one_read *monitor =
 		container_of(wait, struct cachefiles_one_read, monitor);
 	struct cachefiles_object *object;
+	struct fscache_retrieval *op = monitor->op;
 	struct wait_bit_key *key = _key;
 	struct page *page = wait->private;
 
@@ -51,16 +52,22 @@ static int cachefiles_read_waiter(wait_queue_entry_t *wait, unsigned mode,
 	list_del(&wait->entry);
 
 	/* move onto the action list and queue for FS-Cache thread pool */
-	ASSERT(monitor->op);
+	ASSERT(op);
 
-	object = container_of(monitor->op->op.object,
-			      struct cachefiles_object, fscache);
+	/* We need to temporarily bump the usage count as we don't own a ref
+	 * here otherwise cachefiles_read_copier() may free the op between the
+	 * monitor being enqueued on the op->to_do list and the op getting
+	 * enqueued on the work queue.
+	 */
+	fscache_get_retrieval(op);
 
+	object = container_of(op->op.object, struct cachefiles_object, fscache);
 	spin_lock(&object->work_lock);
-	list_add_tail(&monitor->op_link, &monitor->op->to_do);
+	list_add_tail(&monitor->op_link, &op->to_do);
 	spin_unlock(&object->work_lock);
 
-	fscache_enqueue_retrieval(monitor->op);
+	fscache_enqueue_retrieval(op);
+	fscache_put_retrieval(op);
 	return 0;
 }