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1/*
2 * Read-Copy Update mechanism for mutual exclusion
3 *
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
8 *
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * You should have received a copy of the GNU General Public License
15 * along with this program; if not, write to the Free Software
16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
17 *
18 * Copyright IBM Corporation, 2001
19 *
20 * Author: Dipankar Sarma <dipankar@in.ibm.com>
21 *
22 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
23 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
24 * Papers:
25 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
26 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
27 *
28 * For detailed explanation of Read-Copy Update mechanism see -
29 * http://lse.sourceforge.net/locking/rcupdate.html
30 *
31 */
32
33#ifndef __LINUX_RCUPDATE_H
34#define __LINUX_RCUPDATE_H
35
36#ifdef __KERNEL__
37
38#include <linux/cache.h>
39#include <linux/spinlock.h>
40#include <linux/threads.h>
41#include <linux/percpu.h>
42#include <linux/cpumask.h>
43#include <linux/seqlock.h>
44#include <linux/lockdep.h>
45
46/**
47 * struct rcu_head - callback structure for use with RCU
48 * @next: next update requests in a list
49 * @func: actual update function to call after the grace period.
50 */
51struct rcu_head {
52 struct rcu_head *next;
53 void (*func)(struct rcu_head *head);
54};
55
56#ifdef CONFIG_CLASSIC_RCU
57#include <linux/rcuclassic.h>
58#else /* #ifdef CONFIG_CLASSIC_RCU */
59#include <linux/rcupreempt.h>
60#endif /* #else #ifdef CONFIG_CLASSIC_RCU */
61
62#define RCU_HEAD_INIT { .next = NULL, .func = NULL }
63#define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
64#define INIT_RCU_HEAD(ptr) do { \
65 (ptr)->next = NULL; (ptr)->func = NULL; \
66} while (0)
67
68/**
69 * rcu_read_lock - mark the beginning of an RCU read-side critical section.
70 *
71 * When synchronize_rcu() is invoked on one CPU while other CPUs
72 * are within RCU read-side critical sections, then the
73 * synchronize_rcu() is guaranteed to block until after all the other
74 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
75 * on one CPU while other CPUs are within RCU read-side critical
76 * sections, invocation of the corresponding RCU callback is deferred
77 * until after the all the other CPUs exit their critical sections.
78 *
79 * Note, however, that RCU callbacks are permitted to run concurrently
80 * with RCU read-side critical sections. One way that this can happen
81 * is via the following sequence of events: (1) CPU 0 enters an RCU
82 * read-side critical section, (2) CPU 1 invokes call_rcu() to register
83 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
84 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
85 * callback is invoked. This is legal, because the RCU read-side critical
86 * section that was running concurrently with the call_rcu() (and which
87 * therefore might be referencing something that the corresponding RCU
88 * callback would free up) has completed before the corresponding
89 * RCU callback is invoked.
90 *
91 * RCU read-side critical sections may be nested. Any deferred actions
92 * will be deferred until the outermost RCU read-side critical section
93 * completes.
94 *
95 * It is illegal to block while in an RCU read-side critical section.
96 */
97#define rcu_read_lock() __rcu_read_lock()
98
99/**
100 * rcu_read_unlock - marks the end of an RCU read-side critical section.
101 *
102 * See rcu_read_lock() for more information.
103 */
104
105/*
106 * So where is rcu_write_lock()? It does not exist, as there is no
107 * way for writers to lock out RCU readers. This is a feature, not
108 * a bug -- this property is what provides RCU's performance benefits.
109 * Of course, writers must coordinate with each other. The normal
110 * spinlock primitives work well for this, but any other technique may be
111 * used as well. RCU does not care how the writers keep out of each
112 * others' way, as long as they do so.
113 */
114#define rcu_read_unlock() __rcu_read_unlock()
115
116/**
117 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
118 *
119 * This is equivalent of rcu_read_lock(), but to be used when updates
120 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
121 * consider completion of a softirq handler to be a quiescent state,
122 * a process in RCU read-side critical section must be protected by
123 * disabling softirqs. Read-side critical sections in interrupt context
124 * can use just rcu_read_lock().
125 *
126 */
127#define rcu_read_lock_bh() __rcu_read_lock_bh()
128
129/*
130 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
131 *
132 * See rcu_read_lock_bh() for more information.
133 */
134#define rcu_read_unlock_bh() __rcu_read_unlock_bh()
135
136/*
137 * Prevent the compiler from merging or refetching accesses. The compiler
138 * is also forbidden from reordering successive instances of ACCESS_ONCE(),
139 * but only when the compiler is aware of some particular ordering. One way
140 * to make the compiler aware of ordering is to put the two invocations of
141 * ACCESS_ONCE() in different C statements.
142 *
143 * This macro does absolutely -nothing- to prevent the CPU from reordering,
144 * merging, or refetching absolutely anything at any time.
145 */
146#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
147
148/**
149 * rcu_dereference - fetch an RCU-protected pointer in an
150 * RCU read-side critical section. This pointer may later
151 * be safely dereferenced.
152 *
153 * Inserts memory barriers on architectures that require them
154 * (currently only the Alpha), and, more importantly, documents
155 * exactly which pointers are protected by RCU.
156 */
157
158#define rcu_dereference(p) ({ \
159 typeof(p) _________p1 = ACCESS_ONCE(p); \
160 smp_read_barrier_depends(); \
161 (_________p1); \
162 })
163
164/**
165 * rcu_assign_pointer - assign (publicize) a pointer to a newly
166 * initialized structure that will be dereferenced by RCU read-side
167 * critical sections. Returns the value assigned.
168 *
169 * Inserts memory barriers on architectures that require them
170 * (pretty much all of them other than x86), and also prevents
171 * the compiler from reordering the code that initializes the
172 * structure after the pointer assignment. More importantly, this
173 * call documents which pointers will be dereferenced by RCU read-side
174 * code.
175 */
176
177#define rcu_assign_pointer(p, v) \
178 ({ \
179 if (!__builtin_constant_p(v) || \
180 ((v) != NULL)) \
181 smp_wmb(); \
182 (p) = (v); \
183 })
184
185/**
186 * synchronize_sched - block until all CPUs have exited any non-preemptive
187 * kernel code sequences.
188 *
189 * This means that all preempt_disable code sequences, including NMI and
190 * hardware-interrupt handlers, in progress on entry will have completed
191 * before this primitive returns. However, this does not guarantee that
192 * softirq handlers will have completed, since in some kernels, these
193 * handlers can run in process context, and can block.
194 *
195 * This primitive provides the guarantees made by the (now removed)
196 * synchronize_kernel() API. In contrast, synchronize_rcu() only
197 * guarantees that rcu_read_lock() sections will have completed.
198 * In "classic RCU", these two guarantees happen to be one and
199 * the same, but can differ in realtime RCU implementations.
200 */
201#define synchronize_sched() __synchronize_sched()
202
203/**
204 * call_rcu - Queue an RCU callback for invocation after a grace period.
205 * @head: structure to be used for queueing the RCU updates.
206 * @func: actual update function to be invoked after the grace period
207 *
208 * The update function will be invoked some time after a full grace
209 * period elapses, in other words after all currently executing RCU
210 * read-side critical sections have completed. RCU read-side critical
211 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
212 * and may be nested.
213 */
214extern void call_rcu(struct rcu_head *head,
215 void (*func)(struct rcu_head *head));
216
217/**
218 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
219 * @head: structure to be used for queueing the RCU updates.
220 * @func: actual update function to be invoked after the grace period
221 *
222 * The update function will be invoked some time after a full grace
223 * period elapses, in other words after all currently executing RCU
224 * read-side critical sections have completed. call_rcu_bh() assumes
225 * that the read-side critical sections end on completion of a softirq
226 * handler. This means that read-side critical sections in process
227 * context must not be interrupted by softirqs. This interface is to be
228 * used when most of the read-side critical sections are in softirq context.
229 * RCU read-side critical sections are delimited by :
230 * - rcu_read_lock() and rcu_read_unlock(), if in interrupt context.
231 * OR
232 * - rcu_read_lock_bh() and rcu_read_unlock_bh(), if in process context.
233 * These may be nested.
234 */
235extern void call_rcu_bh(struct rcu_head *head,
236 void (*func)(struct rcu_head *head));
237
238/* Exported common interfaces */
239extern void synchronize_rcu(void);
240extern void rcu_barrier(void);
241extern long rcu_batches_completed(void);
242extern long rcu_batches_completed_bh(void);
243
244/* Internal to kernel */
245extern void rcu_init(void);
246extern int rcu_needs_cpu(int cpu);
247
248#endif /* __KERNEL__ */
249#endif /* __LINUX_RCUPDATE_H */