7 * Userspace RCU header.
9 * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu.h for linking
10 * dynamically with the userspace rcu library.
12 * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
13 * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
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.
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.
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
29 * IBM's contributions to this file may be relicensed under LGPLv2 or later.
40 * Identify a shared load. A smp_rmc() or smp_mc() should come before the load.
42 #define _LOAD_SHARED(p) ACCESS_ONCE(p)
45 * Load a data from shared memory, doing a cache flush if required.
47 #define LOAD_SHARED(p) \
54 * Identify a shared store. A smp_wmc() or smp_mc() should follow the store.
56 #define _STORE_SHARED(x, v) ({ ACCESS_ONCE(x) = (v); })
59 * Store v into x, where x is located in shared memory. Performs the required
60 * cache flush after writing. Returns v.
62 #define STORE_SHARED(x, v) \
64 _STORE_SHARED(x, v); \
70 * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
71 * into a RCU read-side critical section. The pointer can later be safely
72 * dereferenced within the critical section.
74 * This ensures that the pointer copy is invariant thorough the whole critical
77 * Inserts memory barriers on architectures that require them (currently only
78 * Alpha) and documents which pointers are protected by RCU.
80 * The compiler memory barrier in LOAD_SHARED() ensures that value-speculative
81 * optimizations (e.g. VSS: Value Speculation Scheduling) does not perform the
82 * data read before the pointer read by speculating the value of the pointer.
83 * Correct ordering is ensured because the pointer is read as a volatile access.
84 * This acts as a global side-effect operation, which forbids reordering of
85 * dependent memory operations. Note that such concern about dependency-breaking
86 * optimizations will eventually be taken care of by the "memory_order_consume"
87 * addition to forthcoming C++ standard.
89 * Should match rcu_assign_pointer() or rcu_xchg_pointer().
92 #define _rcu_dereference(p) ({ \
93 typeof(p) _________p1 = LOAD_SHARED(p); \
94 smp_read_barrier_depends(); \
99 * This code section can only be included in LGPL 2.1 compatible source code.
100 * See below for the function call wrappers which can be used in code meant to
101 * be only linked with the Userspace RCU library. This comes with a small
102 * performance degradation on the read-side due to the added function calls.
103 * This is required to permit relinking with newer versions of the library.
107 * The signal number used by the RCU library can be overridden with
108 * -DSIGURCU= when compiling the library.
111 #define SIGURCU SIGUSR1
115 * If a reader is really non-cooperative and refuses to commit its
116 * urcu_active_readers count to memory (there is no barrier in the reader
117 * per-se), kick it after a few loops waiting for it.
119 #define KICK_READER_LOOPS 10000
122 * Active attempts to check for reader Q.S. before calling sched_yield().
124 #define RCU_QS_ACTIVE_ATTEMPTS 100
127 #define rcu_assert(args...) assert(args)
129 #define rcu_assert(args...)
138 #define YIELD_READ (1 << 0)
139 #define YIELD_WRITE (1 << 1)
142 * Updates without URCU_MB are much slower. Account this in
146 /* maximum sleep delay, in us */
149 #define MAX_SLEEP 30000
152 extern unsigned int yield_active
;
153 extern unsigned int __thread rand_yield
;
155 static inline void debug_yield_read(void)
157 if (yield_active
& YIELD_READ
)
158 if (rand_r(&rand_yield
) & 0x1)
159 usleep(rand_r(&rand_yield
) % MAX_SLEEP
);
162 static inline void debug_yield_write(void)
164 if (yield_active
& YIELD_WRITE
)
165 if (rand_r(&rand_yield
) & 0x1)
166 usleep(rand_r(&rand_yield
) % MAX_SLEEP
);
169 static inline void debug_yield_init(void)
171 rand_yield
= time(NULL
) ^ pthread_self();
174 static inline void debug_yield_read(void)
178 static inline void debug_yield_write(void)
182 static inline void debug_yield_init(void)
189 static inline void reader_barrier()
194 static inline void reader_barrier()
201 * The trick here is that RCU_GP_CTR_BIT must be a multiple of 8 so we can use a
202 * full 8-bits, 16-bits or 32-bits bitmask for the lower order bits.
204 #define RCU_GP_COUNT (1UL << 0)
205 /* Use the amount of bits equal to half of the architecture long size */
206 #define RCU_GP_CTR_BIT (1UL << (sizeof(long) << 2))
207 #define RCU_GP_CTR_NEST_MASK (RCU_GP_CTR_BIT - 1)
208 #define RCU_GP_ONGOING (RCU_GP_CTR_BIT << 1)
211 * Global quiescent period counter with low-order bits unused.
212 * Using a int rather than a char to eliminate false register dependencies
213 * causing stalls on some architectures.
215 extern long urcu_gp_ctr
;
217 struct urcu_reader_status
{
222 extern struct urcu_reader_status __thread urcu_reader_status
;
224 static inline int rcu_old_gp_ongoing(long *value
)
231 * Make sure both tests below are done on the same version of *value
232 * to insure consistency.
234 v
= LOAD_SHARED(*value
);
235 return (v
& RCU_GP_CTR_NEST_MASK
) &&
236 ((v
^ urcu_gp_ctr
) & RCU_GP_CTR_BIT
);
239 static inline void _rcu_read_lock(void)
243 tmp
= urcu_reader_status
.active_readers
;
244 /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
245 if (likely(!(tmp
& RCU_GP_CTR_NEST_MASK
))) {
247 * volatile accesses can be reordered and optimized when within
248 * the same statement.
250 if (unlikely((gp_ctr
= _LOAD_SHARED(urcu_gp_ctr
))
252 unlikely(LOAD_SHARED(urcu_reader_status
.gp_waiting
))) {
254 gp_ctr
= _LOAD_SHARED(urcu_gp_ctr
);
256 _STORE_SHARED(urcu_reader_status
.active_readers
, gp_ctr
);
258 * Set active readers count for outermost nesting level before
259 * accessing the pointer. See force_mb_all_threads().
263 _STORE_SHARED(urcu_reader_status
.active_readers
,
268 static inline void _rcu_read_unlock(void)
271 * Finish using rcu before decrementing the pointer.
272 * See force_mb_all_threads().
273 * Formally only needed for outermost nesting level, but leave barrier
274 * in place for nested unlocks to remove a branch from the common case
278 _STORE_SHARED(urcu_reader_status
.active_readers
,
279 urcu_reader_status
.active_readers
- RCU_GP_COUNT
);
283 * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
284 * meant to be read by RCU read-side critical sections. Returns the assigned
287 * Documents which pointers will be dereferenced by RCU read-side critical
288 * sections and adds the required memory barriers on architectures requiring
289 * them. It also makes sure the compiler does not reorder code initializing the
290 * data structure before its publication.
292 * Should match rcu_dereference_pointer().
295 #define _rcu_assign_pointer(p, v) \
297 if (!__builtin_constant_p(v) || \
300 STORE_SHARED(p, v); \
304 * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
305 * is as expected by "old". If succeeds, returns the previous pointer to the
306 * data structure, which can be safely freed after waiting for a quiescent state
307 * using synchronize_rcu(). If fails (unexpected value), returns old (which
308 * should not be freed !).
311 #define _rcu_cmpxchg_pointer(p, old, _new) \
313 if (!__builtin_constant_p(_new) || \
316 cmpxchg(p, old, _new); \
320 * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
321 * pointer to the data structure, which can be safely freed after waiting for a
322 * quiescent state using synchronize_rcu().
325 #define _rcu_xchg_pointer(p, v) \
327 if (!__builtin_constant_p(v) || \
334 * Exchanges the pointer and waits for quiescent state.
335 * The pointer returned can be freed.
337 #define _rcu_publish_content(p, v) \
340 oldptr = _rcu_xchg_pointer(p, v); \
345 #endif /* _URCU_STATIC_H */