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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 (C) IBM Corporation, 2001
19 *
20 * Author: Dipankar Sarma <dipankar@in.ibm.com>
21 *
22 * Based on the original work by Paul McKenney <paul.mckenney@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
45/**
46 * struct rcu_head - callback structure for use with RCU
47 * @next: next update requests in a list
48 * @func: actual update function to call after the grace period.
49 */
50struct rcu_head {
51 struct rcu_head *next;
52 void (*func)(struct rcu_head *head);
53};
54
55#define RCU_HEAD_INIT { .next = NULL, .func = NULL }
56#define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
57#define INIT_RCU_HEAD(ptr) do { \
58 (ptr)->next = NULL; (ptr)->func = NULL; \
59} while (0)
60
61
62
63/* Global control variables for rcupdate callback mechanism. */
64struct rcu_ctrlblk {
65 long cur; /* Current batch number. */
66 long completed; /* Number of the last completed batch */
67 int next_pending; /* Is the next batch already waiting? */
68} ____cacheline_maxaligned_in_smp;
69
70/* Is batch a before batch b ? */
71static inline int rcu_batch_before(long a, long b)
72{
73 return (a - b) < 0;
74}
75
76/* Is batch a after batch b ? */
77static inline int rcu_batch_after(long a, long b)
78{
79 return (a - b) > 0;
80}
81
82/*
83 * Per-CPU data for Read-Copy UPdate.
84 * nxtlist - new callbacks are added here
85 * curlist - current batch for which quiescent cycle started if any
86 */
87struct rcu_data {
88 /* 1) quiescent state handling : */
89 long quiescbatch; /* Batch # for grace period */
90 int passed_quiesc; /* User-mode/idle loop etc. */
91 int qs_pending; /* core waits for quiesc state */
92
93 /* 2) batch handling */
94 long batch; /* Batch # for current RCU batch */
95 struct rcu_head *nxtlist;
96 struct rcu_head **nxttail;
97 long count; /* # of queued items */
98 struct rcu_head *curlist;
99 struct rcu_head **curtail;
100 struct rcu_head *donelist;
101 struct rcu_head **donetail;
102 int cpu;
103};
104
105DECLARE_PER_CPU(struct rcu_data, rcu_data);
106DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);
107extern struct rcu_ctrlblk rcu_ctrlblk;
108extern struct rcu_ctrlblk rcu_bh_ctrlblk;
109
110/*
111 * Increment the quiescent state counter.
112 * The counter is a bit degenerated: We do not need to know
113 * how many quiescent states passed, just if there was at least
114 * one since the start of the grace period. Thus just a flag.
115 */
116static inline void rcu_qsctr_inc(int cpu)
117{
118 struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
119 rdp->passed_quiesc = 1;
120}
121static inline void rcu_bh_qsctr_inc(int cpu)
122{
123 struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
124 rdp->passed_quiesc = 1;
125}
126
127static inline int __rcu_pending(struct rcu_ctrlblk *rcp,
128 struct rcu_data *rdp)
129{
130 /* This cpu has pending rcu entries and the grace period
131 * for them has completed.
132 */
133 if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch))
134 return 1;
135
136 /* This cpu has no pending entries, but there are new entries */
137 if (!rdp->curlist && rdp->nxtlist)
138 return 1;
139
140 /* This cpu has finished callbacks to invoke */
141 if (rdp->donelist)
142 return 1;
143
144 /* The rcu core waits for a quiescent state from the cpu */
145 if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
146 return 1;
147
148 /* nothing to do */
149 return 0;
150}
151
152static inline int rcu_pending(int cpu)
153{
154 return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
155 __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
156}
157
158/**
159 * rcu_read_lock - mark the beginning of an RCU read-side critical section.
160 *
161 * When synchronize_rcu() is invoked on one CPU while other CPUs
162 * are within RCU read-side critical sections, then the
163 * synchronize_rcu() is guaranteed to block until after all the other
164 * CPUs exit their critical sections. Similarly, if call_rcu() is invoked
165 * on one CPU while other CPUs are within RCU read-side critical
166 * sections, invocation of the corresponding RCU callback is deferred
167 * until after the all the other CPUs exit their critical sections.
168 *
169 * Note, however, that RCU callbacks are permitted to run concurrently
170 * with RCU read-side critical sections. One way that this can happen
171 * is via the following sequence of events: (1) CPU 0 enters an RCU
172 * read-side critical section, (2) CPU 1 invokes call_rcu() to register
173 * an RCU callback, (3) CPU 0 exits the RCU read-side critical section,
174 * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU
175 * callback is invoked. This is legal, because the RCU read-side critical
176 * section that was running concurrently with the call_rcu() (and which
177 * therefore might be referencing something that the corresponding RCU
178 * callback would free up) has completed before the corresponding
179 * RCU callback is invoked.
180 *
181 * RCU read-side critical sections may be nested. Any deferred actions
182 * will be deferred until the outermost RCU read-side critical section
183 * completes.
184 *
185 * It is illegal to block while in an RCU read-side critical section.
186 */
187#define rcu_read_lock() preempt_disable()
188
189/**
190 * rcu_read_unlock - marks the end of an RCU read-side critical section.
191 *
192 * See rcu_read_lock() for more information.
193 */
194#define rcu_read_unlock() preempt_enable()
195
196/*
197 * So where is rcu_write_lock()? It does not exist, as there is no
198 * way for writers to lock out RCU readers. This is a feature, not
199 * a bug -- this property is what provides RCU's performance benefits.
200 * Of course, writers must coordinate with each other. The normal
201 * spinlock primitives work well for this, but any other technique may be
202 * used as well. RCU does not care how the writers keep out of each
203 * others' way, as long as they do so.
204 */
205
206/**
207 * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section
208 *
209 * This is equivalent of rcu_read_lock(), but to be used when updates
210 * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
211 * consider completion of a softirq handler to be a quiescent state,
212 * a process in RCU read-side critical section must be protected by
213 * disabling softirqs. Read-side critical sections in interrupt context
214 * can use just rcu_read_lock().
215 *
216 */
217#define rcu_read_lock_bh() local_bh_disable()
218
219/*
220 * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
221 *
222 * See rcu_read_lock_bh() for more information.
223 */
224#define rcu_read_unlock_bh() local_bh_enable()
225
226/**
227 * rcu_dereference - fetch an RCU-protected pointer in an
228 * RCU read-side critical section. This pointer may later
229 * be safely dereferenced.
230 *
231 * Inserts memory barriers on architectures that require them
232 * (currently only the Alpha), and, more importantly, documents
233 * exactly which pointers are protected by RCU.
234 */
235
236#define rcu_dereference(p) ({ \
237 typeof(p) _________p1 = p; \
238 smp_read_barrier_depends(); \
239 (_________p1); \
240 })
241
242/**
243 * rcu_assign_pointer - assign (publicize) a pointer to a newly
244 * initialized structure that will be dereferenced by RCU read-side
245 * critical sections. Returns the value assigned.
246 *
247 * Inserts memory barriers on architectures that require them
248 * (pretty much all of them other than x86), and also prevents
249 * the compiler from reordering the code that initializes the
250 * structure after the pointer assignment. More importantly, this
251 * call documents which pointers will be dereferenced by RCU read-side
252 * code.
253 */
254
255#define rcu_assign_pointer(p, v) ({ \
256 smp_wmb(); \
257 (p) = (v); \
258 })
259
260/**
261 * synchronize_sched - block until all CPUs have exited any non-preemptive
262 * kernel code sequences.
263 *
264 * This means that all preempt_disable code sequences, including NMI and
265 * hardware-interrupt handlers, in progress on entry will have completed
266 * before this primitive returns. However, this does not guarantee that
267 * softirq handlers will have completed, since in some kernels
268 *
269 * This primitive provides the guarantees made by the (deprecated)
270 * synchronize_kernel() API. In contrast, synchronize_rcu() only
271 * guarantees that rcu_read_lock() sections will have completed.
272 */
273#define synchronize_sched() synchronize_rcu()
274
275extern void rcu_init(void);
276extern void rcu_check_callbacks(int cpu, int user);
277extern void rcu_restart_cpu(int cpu);
278
279/* Exported interfaces */
280extern void FASTCALL(call_rcu(struct rcu_head *head,
281 void (*func)(struct rcu_head *head)));
282extern void FASTCALL(call_rcu_bh(struct rcu_head *head,
283 void (*func)(struct rcu_head *head)));
284extern __deprecated_for_modules void synchronize_kernel(void);
285extern void synchronize_rcu(void);
286void synchronize_idle(void);
287
288#endif /* __KERNEL__ */
289#endif /* __LINUX_RCUPDATE_H */