Detect RCU read-side underflows
[urcu.git] / urcu / static / urcu-bp.h
1 #ifndef _URCU_BP_STATIC_H
2 #define _URCU_BP_STATIC_H
3
4 /*
5 * urcu-bp-static.h
6 *
7 * Userspace RCU header.
8 *
9 * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
10 * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
11 *
12 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
13 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
14 *
15 * This library is free software; you can redistribute it and/or
16 * modify it under the terms of the GNU Lesser General Public
17 * License as published by the Free Software Foundation; either
18 * version 2.1 of the License, or (at your option) any later version.
19 *
20 * This library is distributed in the hope that it will be useful,
21 * but WITHOUT ANY WARRANTY; without even the implied warranty of
22 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
23 * Lesser General Public License for more details.
24 *
25 * You should have received a copy of the GNU Lesser General Public
26 * License along with this library; if not, write to the Free Software
27 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
28 *
29 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
30 */
31
32 #include <stdlib.h>
33 #include <pthread.h>
34 #include <unistd.h>
35
36 #include <urcu/compiler.h>
37 #include <urcu/arch.h>
38 #include <urcu/system.h>
39 #include <urcu/uatomic.h>
40 #include <urcu/list.h>
41 #include <urcu/tls-compat.h>
42 #include <urcu/debug.h>
43
44 /*
45 * This code section can only be included in LGPL 2.1 compatible source code.
46 * See below for the function call wrappers which can be used in code meant to
47 * be only linked with the Userspace RCU library. This comes with a small
48 * performance degradation on the read-side due to the added function calls.
49 * This is required to permit relinking with newer versions of the library.
50 */
51
52 #ifdef __cplusplus
53 extern "C" {
54 #endif
55
56 enum rcu_state {
57 RCU_READER_ACTIVE_CURRENT,
58 RCU_READER_ACTIVE_OLD,
59 RCU_READER_INACTIVE,
60 };
61
62 /*
63 * The trick here is that RCU_GP_CTR_PHASE must be a multiple of 8 so we can use a
64 * full 8-bits, 16-bits or 32-bits bitmask for the lower order bits.
65 */
66 #define RCU_GP_COUNT (1UL << 0)
67 /* Use the amount of bits equal to half of the architecture long size */
68 #define RCU_GP_CTR_PHASE (1UL << (sizeof(long) << 2))
69 #define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_PHASE - 1)
70
71 /*
72 * Used internally by _rcu_read_lock.
73 */
74 extern void rcu_bp_register(void);
75
76 struct rcu_gp {
77 /*
78 * Global grace period counter.
79 * Contains the current RCU_GP_CTR_PHASE.
80 * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
81 * Written to only by writer with mutex taken.
82 * Read by both writer and readers.
83 */
84 unsigned long ctr;
85 } __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
86
87 extern struct rcu_gp rcu_gp;
88
89 struct rcu_reader {
90 /* Data used by both reader and synchronize_rcu() */
91 unsigned long ctr;
92 /* Data used for registry */
93 struct cds_list_head node __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
94 pthread_t tid;
95 int alloc; /* registry entry allocated */
96 };
97
98 /*
99 * Bulletproof version keeps a pointer to a registry not part of the TLS.
100 * Adds a pointer dereference on the read-side, but won't require to unregister
101 * the reader thread.
102 */
103 extern DECLARE_URCU_TLS(struct rcu_reader *, rcu_reader);
104
105 static inline enum rcu_state rcu_reader_state(unsigned long *ctr)
106 {
107 unsigned long v;
108
109 if (ctr == NULL)
110 return RCU_READER_INACTIVE;
111 /*
112 * Make sure both tests below are done on the same version of *value
113 * to insure consistency.
114 */
115 v = CMM_LOAD_SHARED(*ctr);
116 if (!(v & RCU_GP_CTR_NEST_MASK))
117 return RCU_READER_INACTIVE;
118 if (!((v ^ rcu_gp.ctr) & RCU_GP_CTR_PHASE))
119 return RCU_READER_ACTIVE_CURRENT;
120 return RCU_READER_ACTIVE_OLD;
121 }
122
123 /*
124 * Helper for _rcu_read_lock(). The format of rcu_gp.ctr (as well as
125 * the per-thread rcu_reader.ctr) has the upper bits containing a count of
126 * _rcu_read_lock() nesting, and a lower-order bit that contains either zero
127 * or RCU_GP_CTR_PHASE. The smp_mb_slave() ensures that the accesses in
128 * _rcu_read_lock() happen before the subsequent read-side critical section.
129 */
130 static inline void _rcu_read_lock_update(unsigned long tmp)
131 {
132 if (caa_likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
133 _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, _CMM_LOAD_SHARED(rcu_gp.ctr));
134 cmm_smp_mb();
135 } else
136 _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, tmp + RCU_GP_COUNT);
137 }
138
139 /*
140 * Enter an RCU read-side critical section.
141 *
142 * The first cmm_barrier() call ensures that the compiler does not reorder
143 * the body of _rcu_read_lock() with a mutex.
144 *
145 * This function and its helper are both less than 10 lines long. The
146 * intent is that this function meets the 10-line criterion in LGPL,
147 * allowing this function to be invoked directly from non-LGPL code.
148 */
149 static inline void _rcu_read_lock(void)
150 {
151 unsigned long tmp;
152
153 if (caa_unlikely(!URCU_TLS(rcu_reader)))
154 rcu_bp_register(); /* If not yet registered. */
155 cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
156 tmp = URCU_TLS(rcu_reader)->ctr;
157 _rcu_read_lock_update(tmp);
158 }
159
160 /*
161 * Exit an RCU read-side critical section. This function is less than
162 * 10 lines of code, and is intended to be usable by non-LGPL code, as
163 * called out in LGPL.
164 */
165 static inline void _rcu_read_unlock(void)
166 {
167 unsigned long tmp;
168
169 tmp = URCU_TLS(rcu_reader)->ctr;
170 urcu_assert(tmp & RCU_GP_CTR_NEST_MASK);
171 /* Finish using rcu before decrementing the pointer. */
172 cmm_smp_mb();
173 _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, tmp - RCU_GP_COUNT);
174 cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
175 }
176
177 /*
178 * Returns whether within a RCU read-side critical section.
179 *
180 * This function is less than 10 lines long. The intent is that this
181 * function meets the 10-line criterion for LGPL, allowing this function
182 * to be invoked directly from non-LGPL code.
183 */
184 static inline int _rcu_read_ongoing(void)
185 {
186 if (caa_unlikely(!URCU_TLS(rcu_reader)))
187 rcu_bp_register(); /* If not yet registered. */
188 return URCU_TLS(rcu_reader)->ctr & RCU_GP_CTR_NEST_MASK;
189 }
190
191 #ifdef __cplusplus
192 }
193 #endif
194
195 #endif /* _URCU_BP_STATIC_H */
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