docs: refcount_t documentation

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===================================

refcount_t API compared to atomic_t

===================================

.. contents:: :local:

Introduction

============

The goal of refcount_t API is to provide a minimal API for implementing

an object's reference counters. While a generic architecture-independent

implementation from lib/refcount.c uses atomic operations underneath,

there are a number of differences between some of the ``refcount_*()`` and

``atomic_*()`` functions with regards to the memory ordering guarantees.

This document outlines the differences and provides respective examples

in order to help maintainers validate their code against the change in

these memory ordering guarantees.

The terms used through this document try to follow the formal LKMM defined in

github.com/aparri/memory-model/blob/master/Documentation/explanation.txt

memory-barriers.txt and atomic_t.txt provide more background to the

memory ordering in general and for atomic operations specifically.

Relevant types of memory ordering

=================================

.. note:: The following section only covers some of the memory

   ordering types that are relevant for the atomics and reference

   counters and used through this document. For a much broader picture

   please consult memory-barriers.txt document.

In the absence of any memory ordering guarantees (i.e. fully unordered)

atomics & refcounters only provide atomicity and

program order (po) relation (on the same CPU). It guarantees that

each ``atomic_*()`` and ``refcount_*()`` operation is atomic and instructions

are executed in program order on a single CPU.

This is implemented using :c:func:`READ_ONCE`/:c:func:`WRITE_ONCE` and

compare-and-swap primitives.

A strong (full) memory ordering guarantees that all prior loads and

stores (all po-earlier instructions) on the same CPU are completed

before any po-later instruction is executed on the same CPU.

It also guarantees that all po-earlier stores on the same CPU

and all propagated stores from other CPUs must propagate to all

other CPUs before any po-later instruction is executed on the original

CPU (A-cumulative property). This is implemented using :c:func:`smp_mb`.

A RELEASE memory ordering guarantees that all prior loads and

stores (all po-earlier instructions) on the same CPU are completed

before the operation. It also guarantees that all po-earlier

stores on the same CPU and all propagated stores from other CPUs

must propagate to all other CPUs before the release operation

(A-cumulative property). This is implemented using

:c:func:`smp_store_release`.

A control dependency (on success) for refcounters guarantees that

if a reference for an object was successfully obtained (reference

counter increment or addition happened, function returned true),

then further stores are ordered against this operation.

Control dependency on stores are not implemented using any explicit

barriers, but rely on CPU not to speculate on stores. This is only

a single CPU relation and provides no guarantees for other CPUs.

Comparison of functions

=======================

case 1) - non-"Read/Modify/Write" (RMW) ops

-------------------------------------------

Function changes:

 * :c:func:`atomic_set` --> :c:func:`refcount_set`

 * :c:func:`atomic_read` --> :c:func:`refcount_read`

Memory ordering guarantee changes:

 * none (both fully unordered)

case 2) - increment-based ops that return no value

--------------------------------------------------

Function changes:

 * :c:func:`atomic_inc` --> :c:func:`refcount_inc`

 * :c:func:`atomic_add` --> :c:func:`refcount_add`

Memory ordering guarantee changes:

 * none (both fully unordered)

case 3) - decrement-based RMW ops that return no value

------------------------------------------------------

Function changes:

 * :c:func:`atomic_dec` --> :c:func:`refcount_dec`

Memory ordering guarantee changes:

 * fully unordered --> RELEASE ordering

case 4) - increment-based RMW ops that return a value

-----------------------------------------------------

Function changes:

 * :c:func:`atomic_inc_not_zero` --> :c:func:`refcount_inc_not_zero`

 * no atomic counterpart --> :c:func:`refcount_add_not_zero`

Memory ordering guarantees changes:

 * fully ordered --> control dependency on success for stores

.. note:: We really assume here that necessary ordering is provided as a

   result of obtaining pointer to the object!

case 5) - decrement-based RMW ops that return a value

-----------------------------------------------------

Function changes:

 * :c:func:`atomic_dec_and_test` --> :c:func:`refcount_dec_and_test`

 * :c:func:`atomic_sub_and_test` --> :c:func:`refcount_sub_and_test`

 * no atomic counterpart --> :c:func:`refcount_dec_if_one`

 * ``atomic_add_unless(&var, -1, 1)`` --> ``refcount_dec_not_one(&var)``

Memory ordering guarantees changes:

 * fully ordered --> RELEASE ordering + control dependency

.. note:: :c:func:`atomic_add_unless` only provides full order on success.

case 6) - lock-based RMW

------------------------

Function changes:

 * :c:func:`atomic_dec_and_lock` --> :c:func:`refcount_dec_and_lock`

 * :c:func:`atomic_dec_and_mutex_lock` --> :c:func:`refcount_dec_and_mutex_lock`

Memory ordering guarantees changes:

 * fully ordered --> RELEASE ordering + control dependency + hold

   :c:func:`spin_lock` on success

.. kernel-doc:: include/linux/mod_devicetable.h

   :internal:

Atomic and pointer manipulation

-------------------------------

.. kernel-doc:: arch/x86/include/asm/atomic.h

   :internal:

Delaying, scheduling, and timer routines

----------------------------------------

.. kernel-doc:: kernel/kthread.c

   :export:

Reference counting

------------------

.. kernel-doc:: include/linux/refcount.h

   :internal:

.. kernel-doc:: lib/refcount.c

   :export:

Atomics

-------

.. kernel-doc:: arch/x86/include/asm/atomic.h

   :internal:

Kernel objects manipulation

---------------------------

#include <linux/kernel.h>

/**

 * refcount_t - variant of atomic_t specialized for reference counts

 * struct refcount_t - variant of atomic_t specialized for reference counts

 * @refs: atomic_t counter field

 *

 * The counter saturates at UINT_MAX and will not move once