+ * Owner module for this policy descriptor.
+ *
+ * We need to hold a reference to the module whenever we might make use
+ * of any of the module's contents, i.e.
+ * - If one or more instances of the policy are at a state where they
+ * might be handling a request, i.e.
+ * ptlrpc_nrs_pol_state::NRS_POL_STATE_STARTED or
+ * ptlrpc_nrs_pol_state::NRS_POL_STATE_STOPPING as we will have to
+ * call into the policy's ptlrpc_nrs_pol_ops() handlers. A reference
+ * is taken on the module when
+ * \e ptlrpc_nrs_pol_desc::pd_refs becomes 1, and released when it
+ * becomes 0, so that we hold only one reference to the module maximum
+ * at any time.
+ *
+ * We do not need to hold a reference to the module, even though we
+ * might use code and data from the module, in the following cases:
+ * - During external policy registration, because this should happen in
+ * the module's init() function, in which case the module is safe from
+ * removal because a reference is being held on the module by the
+ * kernel, and iirc kmod (and I guess module-init-tools also) will
+ * serialize any racing processes properly anyway.
+ * - During external policy unregistration, because this should happen
+ * in a module's exit() function, and any attempts to start a policy
+ * instance would need to take a reference on the module, and this is
+ * not possible once we have reached the point where the exit()
+ * handler is called.
+ * - During service registration and unregistration, as service setup
+ * and cleanup, and policy registration, unregistration and policy
+ * instance starting, are serialized by \e nrs_core::nrs_mutex, so
+ * as long as users adhere to the convention of registering policies
+ * in init() and unregistering them in module exit() functions, there
+ * should not be a race between these operations.
+ * - During any policy-specific lprocfs operations, because a reference
+ * is held by the kernel on a proc entry that has been entered by a
+ * syscall, so as long as proc entries are removed during unregistration time,
+ * then unregistration and lprocfs operations will be properly
+ * serialized.
+ */
+ cfs_module_t *pd_owner;
+ /**
+ * Bitmask of \e nrs_policy_flags
+ */
+ unsigned pd_flags;
+ /**
+ * # of references on this descriptor
+ */
+ cfs_atomic_t pd_refs;