Add branch prediction, fix xchg for -mtune=core2
[urcu.git] / urcu.h
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1#ifndef _URCU_H
2#define _URCU_H
3
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4/*
5 * urcu.h
6 *
7 * Userspace RCU header
8 *
9 * Copyright February 2009 - Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
10 *
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11 * Credits for Paul e. McKenney <paulmck@linux.vnet.ibm.com>
12 * for inspiration coming from the Linux kernel RCU and rcu-preempt.
13 *
14 * The barrier, mb, rmb, wmb, atomic_inc, smp_read_barrier_depends, ACCESS_ONCE
15 * and rcu_dereference primitives come from the Linux kernel.
16 *
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17 * Distributed under GPLv2
18 */
19
9b171f46 20#define __USE_GNU
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21#include <stdlib.h>
22
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23/* The "volatile" is due to gcc bugs */
24#define barrier() __asm__ __volatile__("": : :"memory")
25
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26#define likely(x) __builtin_expect(!!(x), 1)
27#define unlikely(x) __builtin_expect(!!(x), 0)
28
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29/* x86 32/64 specific */
30#define mb() asm volatile("mfence":::"memory")
31#define rmb() asm volatile("lfence":::"memory")
32#define wmb() asm volatile("sfence" ::: "memory")
33
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34static inline void atomic_inc(int *v)
35{
36 asm volatile("lock; incl %0"
f69f195a 37 : "+m" (*v));
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38}
39
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40#define xchg(ptr, v) \
41 ((__typeof__(*(ptr)))__xchg((unsigned long)(v), (ptr), sizeof(*(ptr))))
42
43struct __xchg_dummy {
44 unsigned long a[100];
45};
46#define __xg(x) ((struct __xchg_dummy *)(x))
47
48/*
49 * Note: no "lock" prefix even on SMP: xchg always implies lock anyway
50 * Note 2: xchg has side effect, so that attribute volatile is necessary,
51 * but generally the primitive is invalid, *ptr is output argument. --ANK
52 */
53static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
54 int size)
55{
56 switch (size) {
57 case 1:
58 asm volatile("xchgb %b0,%1"
59 : "=q" (x)
60 : "m" (*__xg(ptr)), "0" (x)
61 : "memory");
62 break;
63 case 2:
64 asm volatile("xchgw %w0,%1"
65 : "=r" (x)
66 : "m" (*__xg(ptr)), "0" (x)
67 : "memory");
68 break;
69 case 4:
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70 asm volatile("xchgl %k0,%1"
71 : "=r" (x)
72 : "m" (*__xg(ptr)), "0" (x)
73 : "memory");
74 break;
75 case 8:
76 asm volatile("xchgq %0,%1"
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77 : "=r" (x)
78 : "m" (*__xg(ptr)), "0" (x)
79 : "memory");
80 break;
81 }
82 return x;
83}
84
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85/* Nop everywhere except on alpha. */
86#define smp_read_barrier_depends()
87
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88/*
89 * Prevent the compiler from merging or refetching accesses. The compiler
90 * is also forbidden from reordering successive instances of ACCESS_ONCE(),
91 * but only when the compiler is aware of some particular ordering. One way
92 * to make the compiler aware of ordering is to put the two invocations of
93 * ACCESS_ONCE() in different C statements.
94 *
95 * This macro does absolutely -nothing- to prevent the CPU from reordering,
96 * merging, or refetching absolutely anything at any time. Its main intended
97 * use is to mediate communication between process-level code and irq/NMI
98 * handlers, all running on the same CPU.
99 */
100#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
101
102/**
103 * rcu_dereference - fetch an RCU-protected pointer in an
104 * RCU read-side critical section. This pointer may later
105 * be safely dereferenced.
106 *
107 * Inserts memory barriers on architectures that require them
108 * (currently only the Alpha), and, more importantly, documents
109 * exactly which pointers are protected by RCU.
110 */
111
112#define rcu_dereference(p) ({ \
113 typeof(p) _________p1 = ACCESS_ONCE(p); \
114 smp_read_barrier_depends(); \
115 (_________p1); \
116 })
117
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118#define SIGURCU SIGUSR1
119
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120#ifdef DEBUG_YIELD
121#include <sched.h>
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122#include <time.h>
123#include <pthread.h>
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124
125#define YIELD_READ (1 << 0)
126#define YIELD_WRITE (1 << 1)
127
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128extern unsigned int yield_active;
129extern unsigned int __thread rand_yield;
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130
131static inline void debug_yield_read(void)
132{
133 if (yield_active & YIELD_READ)
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134 if (rand_r(&rand_yield) & 0x1)
135 sched_yield();
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136}
137
138static inline void debug_yield_write(void)
139{
140 if (yield_active & YIELD_WRITE)
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141 if (rand_r(&rand_yield) & 0x1)
142 sched_yield();
143}
144
145static inline void debug_yield_init(void)
146{
147 rand_yield = time(NULL) ^ pthread_self();
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148}
149#else
150static inline void debug_yield_read(void)
151{
152}
153
154static inline void debug_yield_write(void)
155{
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156}
157
158static inline void debug_yield_init(void)
159{
160
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161}
162#endif
163
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164/*
165 * Limiting the nesting level to 256 to keep instructions small in the read
166 * fast-path.
167 */
168#define RCU_GP_COUNT (1U << 0)
169#define RCU_GP_CTR_BIT (1U << 8)
170#define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BIT - 1)
171
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172/*
173 * Global quiescent period counter with low-order bits unused.
174 * Using a int rather than a char to eliminate false register dependencies
175 * causing stalls on some architectures.
176 */
1430ee0b 177extern int urcu_gp_ctr;
27b012e2 178
1430ee0b 179extern int __thread urcu_active_readers;
27b012e2 180
1430ee0b 181static inline int rcu_old_gp_ongoing(int *value)
27b012e2 182{
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183 int v;
184
185 if (value == NULL)
186 return 0;
187 debug_yield_write();
188 v = ACCESS_ONCE(*value);
189 debug_yield_write();
190 return (v & RCU_GP_CTR_NEST_MASK) &&
191 ((v ^ ACCESS_ONCE(urcu_gp_ctr)) & RCU_GP_CTR_BIT);
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192}
193
1430ee0b 194static inline void rcu_read_lock(void)
27b012e2 195{
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196 int tmp;
197
cf380c2f 198 debug_yield_read();
1430ee0b 199 tmp = urcu_active_readers;
cf380c2f 200 debug_yield_read();
5b1da0c8 201 if (likely(!(tmp & RCU_GP_CTR_NEST_MASK)))
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202 urcu_active_readers = urcu_gp_ctr + RCU_GP_COUNT;
203 else
204 urcu_active_readers = tmp + RCU_GP_COUNT;
cf380c2f 205 debug_yield_read();
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206 /*
207 * Increment active readers count before accessing the pointer.
208 * See force_mb_all_threads().
209 */
210 barrier();
cf380c2f 211 debug_yield_read();
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212}
213
1430ee0b 214static inline void rcu_read_unlock(void)
27b012e2 215{
cf380c2f 216 debug_yield_read();
27b012e2 217 barrier();
cf380c2f 218 debug_yield_read();
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219 /*
220 * Finish using rcu before decrementing the pointer.
221 * See force_mb_all_threads().
222 */
1430ee0b 223 urcu_active_readers -= RCU_GP_COUNT;
cf380c2f 224 debug_yield_read();
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225}
226
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227/**
228 * rcu_assign_pointer - assign (publicize) a pointer to a newly
229 * initialized structure that will be dereferenced by RCU read-side
230 * critical sections. Returns the value assigned.
231 *
232 * Inserts memory barriers on architectures that require them
233 * (pretty much all of them other than x86), and also prevents
234 * the compiler from reordering the code that initializes the
235 * structure after the pointer assignment. More importantly, this
236 * call documents which pointers will be dereferenced by RCU read-side
237 * code.
238 */
239
240#define rcu_assign_pointer(p, v) \
241 ({ \
242 if (!__builtin_constant_p(v) || \
243 ((v) != NULL)) \
244 wmb(); \
245 (p) = (v); \
246 })
247
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248#define rcu_xchg_pointer(p, v) \
249 ({ \
250 if (!__builtin_constant_p(v) || \
251 ((v) != NULL)) \
252 wmb(); \
253 xchg(p, v); \
254 })
255
e462817e 256extern void synchronize_rcu(void);
27b012e2 257
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258/*
259 * Exchanges the pointer and waits for quiescent state.
260 * The pointer returned can be freed.
261 */
262#define urcu_publish_content(p, v) \
263 ({ \
264 void *oldptr; \
265 debug_yield_write(); \
266 oldptr = rcu_xchg_pointer(p, v); \
267 synchronize_rcu(); \
268 oldptr; \
269 })
270
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271/*
272 * Reader thread registration.
273 */
274extern void urcu_register_thread(void);
5e7e64b9 275extern void urcu_unregister_thread(void);
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276
277#endif /* _URCU_H */
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