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1/* SPDX-License-Identifier: GPL-2.0-only */
2#ifndef _LINUX_RCUREF_H
3#define _LINUX_RCUREF_H
4
5#include <linux/atomic.h>
6#include <linux/bug.h>
7#include <linux/limits.h>
8#include <linux/lockdep.h>
9#include <linux/preempt.h>
10#include <linux/rcupdate.h>
11
12#define RCUREF_ONEREF 0x00000000U
13#define RCUREF_MAXREF 0x7FFFFFFFU
14#define RCUREF_SATURATED 0xA0000000U
15#define RCUREF_RELEASED 0xC0000000U
16#define RCUREF_DEAD 0xE0000000U
17#define RCUREF_NOREF 0xFFFFFFFFU
18
19/**
20 * rcuref_init - Initialize a rcuref reference count with the given reference count
21 * @ref: Pointer to the reference count
22 * @cnt: The initial reference count typically '1'
23 */
24static inline void rcuref_init(rcuref_t *ref, unsigned int cnt)
25{
26 atomic_set(&ref->refcnt, cnt - 1);
27}
28
29/**
30 * rcuref_read - Read the number of held reference counts of a rcuref
31 * @ref: Pointer to the reference count
32 *
33 * Return: The number of held references (0 ... N). The value 0 does not
34 * indicate that it is safe to schedule the object, protected by this reference
35 * counter, for deconstruction.
36 * If you want to know if the reference counter has been marked DEAD (as
37 * signaled by rcuref_put()) please use rcuread_is_dead().
38 */
39static inline unsigned int rcuref_read(rcuref_t *ref)
40{
41 unsigned int c = atomic_read(&ref->refcnt);
42
43 /* Return 0 if within the DEAD zone. */
44 return c >= RCUREF_RELEASED ? 0 : c + 1;
45}
46
47/**
48 * rcuref_is_dead - Check if the rcuref has been already marked dead
49 * @ref: Pointer to the reference count
50 *
51 * Return: True if the object has been marked DEAD. This signals that a previous
52 * invocation of rcuref_put() returned true on this reference counter meaning
53 * the protected object can safely be scheduled for deconstruction.
54 * Otherwise, returns false.
55 */
56static inline bool rcuref_is_dead(rcuref_t *ref)
57{
58 unsigned int c = atomic_read(&ref->refcnt);
59
60 return (c >= RCUREF_RELEASED) && (c < RCUREF_NOREF);
61}
62
63extern __must_check bool rcuref_get_slowpath(rcuref_t *ref);
64
65/**
66 * rcuref_get - Acquire one reference on a rcuref reference count
67 * @ref: Pointer to the reference count
68 *
69 * Similar to atomic_inc_not_zero() but saturates at RCUREF_MAXREF.
70 *
71 * Provides no memory ordering, it is assumed the caller has guaranteed the
72 * object memory to be stable (RCU, etc.). It does provide a control dependency
73 * and thereby orders future stores. See documentation in lib/rcuref.c
74 *
75 * Return:
76 * False if the attempt to acquire a reference failed. This happens
77 * when the last reference has been put already
78 *
79 * True if a reference was successfully acquired
80 */
81static inline __must_check bool rcuref_get(rcuref_t *ref)
82{
83 /*
84 * Unconditionally increase the reference count. The saturation and
85 * dead zones provide enough tolerance for this.
86 */
87 if (likely(!atomic_add_negative_relaxed(1, &ref->refcnt)))
88 return true;
89
90 /* Handle the cases inside the saturation and dead zones */
91 return rcuref_get_slowpath(ref);
92}
93
94extern __must_check bool rcuref_put_slowpath(rcuref_t *ref, unsigned int cnt);
95
96/*
97 * Internal helper. Do not invoke directly.
98 */
99static __always_inline __must_check bool __rcuref_put(rcuref_t *ref)
100{
101 int cnt;
102
103 RCU_LOCKDEP_WARN(!rcu_read_lock_held() && preemptible(),
104 "suspicious rcuref_put_rcusafe() usage");
105 /*
106 * Unconditionally decrease the reference count. The saturation and
107 * dead zones provide enough tolerance for this.
108 */
109 cnt = atomic_sub_return_release(1, &ref->refcnt);
110 if (likely(cnt >= 0))
111 return false;
112
113 /*
114 * Handle the last reference drop and cases inside the saturation
115 * and dead zones.
116 */
117 return rcuref_put_slowpath(ref, cnt);
118}
119
120/**
121 * rcuref_put_rcusafe -- Release one reference for a rcuref reference count RCU safe
122 * @ref: Pointer to the reference count
123 *
124 * Provides release memory ordering, such that prior loads and stores are done
125 * before, and provides an acquire ordering on success such that free()
126 * must come after.
127 *
128 * Can be invoked from contexts, which guarantee that no grace period can
129 * happen which would free the object concurrently if the decrement drops
130 * the last reference and the slowpath races against a concurrent get() and
131 * put() pair. rcu_read_lock()'ed and atomic contexts qualify.
132 *
133 * Return:
134 * True if this was the last reference with no future references
135 * possible. This signals the caller that it can safely release the
136 * object which is protected by the reference counter.
137 *
138 * False if there are still active references or the put() raced
139 * with a concurrent get()/put() pair. Caller is not allowed to
140 * release the protected object.
141 */
142static inline __must_check bool rcuref_put_rcusafe(rcuref_t *ref)
143{
144 return __rcuref_put(ref);
145}
146
147/**
148 * rcuref_put -- Release one reference for a rcuref reference count
149 * @ref: Pointer to the reference count
150 *
151 * Can be invoked from any context.
152 *
153 * Provides release memory ordering, such that prior loads and stores are done
154 * before, and provides an acquire ordering on success such that free()
155 * must come after.
156 *
157 * Return:
158 *
159 * True if this was the last reference with no future references
160 * possible. This signals the caller that it can safely schedule the
161 * object, which is protected by the reference counter, for
162 * deconstruction.
163 *
164 * False if there are still active references or the put() raced
165 * with a concurrent get()/put() pair. Caller is not allowed to
166 * deconstruct the protected object.
167 */
168static inline __must_check bool rcuref_put(rcuref_t *ref)
169{
170 bool released;
171
172 preempt_disable();
173 released = __rcuref_put(ref);
174 preempt_enable();
175 return released;
176}
177
178#endif