urcu-static.h

   1 #ifndef _URCU_STATIC_H
   2 #define _URCU_STATIC_H
   3
   4 /*
   5  * urcu-static.h
   6  *
   7  * Userspace RCU header.
   8  *
   9  * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu.h for linking
  10  * dynamically with the userspace rcu library.
  11  *
  12  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
  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 <syscall.h>
  35 #include <unistd.h>
  36
  37 #include <urcu/compiler.h>
  38 #include <urcu/arch.h>
  39
  40 /*
  41  * Identify a shared load. A smp_rmc() or smp_mc() should come before the load.
  42  */
  43 #define _LOAD_SHARED(p)        ACCESS_ONCE(p)
  44
  45 /*
  46  * Load a data from shared memory, doing a cache flush if required.
  47  */
  48 #define LOAD_SHARED(p)                  \
  49         ({                              \
  50                 smp_rmc();              \
  51                 _LOAD_SHARED(p);        \
  52         })
  53
  54 /*
  55  * Identify a shared store. A smp_wmc() or smp_mc() should follow the store.
  56  */
  57 #define _STORE_SHARED(x, v)     ({ ACCESS_ONCE(x) = (v); })
  58
  59 /*
  60  * Store v into x, where x is located in shared memory. Performs the required
  61  * cache flush after writing. Returns v.
  62  */
  63 #define STORE_SHARED(x, v)              \
  64         ({                              \
  65                 _STORE_SHARED(x, v);    \
  66                 smp_wmc();              \
  67                 (v);                    \
  68         })
  69
  70 /**
  71  * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
  72  * into a RCU read-side critical section. The pointer can later be safely
  73  * dereferenced within the critical section.
  74  *
  75  * This ensures that the pointer copy is invariant thorough the whole critical
  76  * section.
  77  *
  78  * Inserts memory barriers on architectures that require them (currently only
  79  * Alpha) and documents which pointers are protected by RCU.
  80  *
  81  * The compiler memory barrier in LOAD_SHARED() ensures that value-speculative
  82  * optimizations (e.g. VSS: Value Speculation Scheduling) does not perform the
  83  * data read before the pointer read by speculating the value of the pointer.
  84  * Correct ordering is ensured because the pointer is read as a volatile access.
  85  * This acts as a global side-effect operation, which forbids reordering of
  86  * dependent memory operations. Note that such concern about dependency-breaking
  87  * optimizations will eventually be taken care of by the "memory_order_consume"
  88  * addition to forthcoming C++ standard.
  89  *
  90  * Should match rcu_assign_pointer() or rcu_xchg_pointer().
  91  */
  92
  93 #define _rcu_dereference(p)     ({                                      \
  94                                 typeof(p) _________p1 = LOAD_SHARED(p); \
  95                                 smp_read_barrier_depends();             \
  96                                 (_________p1);                          \
  97                                 })
  98
  99 #define futex(...)              syscall(__NR_futex, __VA_ARGS__)
 100 #define FUTEX_WAIT              0
 101 #define FUTEX_WAKE              1
 102
 103 /*
 104  * This code section can only be included in LGPL 2.1 compatible source code.
 105  * See below for the function call wrappers which can be used in code meant to
 106  * be only linked with the Userspace RCU library. This comes with a small
 107  * performance degradation on the read-side due to the added function calls.
 108  * This is required to permit relinking with newer versions of the library.
 109  */
 110
 111 /*
 112  * The signal number used by the RCU library can be overridden with
 113  * -DSIGURCU= when compiling the library.
 114  */
 115 #ifndef SIGURCU
 116 #define SIGURCU SIGUSR1
 117 #endif
 118
 119 /*
 120  * If a reader is really non-cooperative and refuses to commit its
 121  * urcu_active_readers count to memory (there is no barrier in the reader
 122  * per-se), kick it after a few loops waiting for it.
 123  */
 124 #define KICK_READER_LOOPS 10000
 125
 126 /*
 127  * Active attempts to check for reader Q.S. before calling futex().
 128  */
 129 #define RCU_QS_ACTIVE_ATTEMPTS 100
 130
 131 #ifdef DEBUG_RCU
 132 #define rcu_assert(args...)     assert(args)
 133 #else
 134 #define rcu_assert(args...)
 135 #endif
 136
 137 #ifdef DEBUG_YIELD
 138 #include <sched.h>
 139 #include <time.h>
 140 #include <pthread.h>
 141 #include <unistd.h>
 142
 143 #define YIELD_READ      (1 << 0)
 144 #define YIELD_WRITE     (1 << 1)
 145
 146 /*
 147  * Updates without URCU_MB are much slower. Account this in
 148  * the delay.
 149  */
 150 #ifdef URCU_MB
 151 /* maximum sleep delay, in us */
 152 #define MAX_SLEEP 50
 153 #else
 154 #define MAX_SLEEP 30000
 155 #endif
 156
 157 extern unsigned int yield_active;
 158 extern unsigned int __thread rand_yield;
 159
 160 static inline void debug_yield_read(void)
 161 {
 162         if (yield_active & YIELD_READ)
 163                 if (rand_r(&rand_yield) & 0x1)
 164                         usleep(rand_r(&rand_yield) % MAX_SLEEP);
 165 }
 166
 167 static inline void debug_yield_write(void)
 168 {
 169         if (yield_active & YIELD_WRITE)
 170                 if (rand_r(&rand_yield) & 0x1)
 171                         usleep(rand_r(&rand_yield) % MAX_SLEEP);
 172 }
 173
 174 static inline void debug_yield_init(void)
 175 {
 176         rand_yield = time(NULL) ^ pthread_self();
 177 }
 178 #else
 179 static inline void debug_yield_read(void)
 180 {
 181 }
 182
 183 static inline void debug_yield_write(void)
 184 {
 185 }
 186
 187 static inline void debug_yield_init(void)
 188 {
 189
 190 }
 191 #endif
 192
 193 #ifdef URCU_MB
 194 static inline void reader_barrier()
 195 {
 196         smp_mb();
 197 }
 198 #else
 199 static inline void reader_barrier()
 200 {
 201         barrier();
 202 }
 203 #endif
 204
 205 /*
 206  * The trick here is that RCU_GP_CTR_BIT must be a multiple of 8 so we can use a
 207  * full 8-bits, 16-bits or 32-bits bitmask for the lower order bits.
 208  */
 209 #define RCU_GP_COUNT            (1UL << 0)
 210 /* Use the amount of bits equal to half of the architecture long size */
 211 #define RCU_GP_CTR_BIT          (1UL << (sizeof(long) << 2))
 212 #define RCU_GP_CTR_NEST_MASK    (RCU_GP_CTR_BIT - 1)
 213
 214 /*
 215  * Global quiescent period counter with low-order bits unused.
 216  * Using a int rather than a char to eliminate false register dependencies
 217  * causing stalls on some architectures.
 218  */
 219 extern long urcu_gp_ctr;
 220
 221 extern long __thread urcu_active_readers;
 222
 223 extern int gp_futex;
 224
 225 /*
 226  * Wake-up waiting synchronize_rcu(). Called from many concurrent threads.
 227  */
 228 static inline void wake_up_gp(void)
 229 {
 230         if (unlikely(uatomic_read(&gp_futex) == -1)) {
 231                 uatomic_set(&gp_futex, 0);
 232                 futex(&gp_futex, FUTEX_WAKE, 1,
 233                       NULL, NULL, 0);
 234         }
 235 }
 236
 237 static inline int rcu_old_gp_ongoing(long *value)
 238 {
 239         long v;
 240
 241         if (value == NULL)
 242                 return 0;
 243         /*
 244          * Make sure both tests below are done on the same version of *value
 245          * to insure consistency.
 246          */
 247         v = LOAD_SHARED(*value);
 248         return (v & RCU_GP_CTR_NEST_MASK) &&
 249                  ((v ^ urcu_gp_ctr) & RCU_GP_CTR_BIT);
 250 }
 251
 252 static inline void _rcu_read_lock(void)
 253 {
 254         long tmp;
 255
 256         tmp = urcu_active_readers;
 257         /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
 258         if (likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
 259                 _STORE_SHARED(urcu_active_readers, _LOAD_SHARED(urcu_gp_ctr));
 260                 /*
 261                  * Set active readers count for outermost nesting level before
 262                  * accessing the pointer. See force_mb_all_threads().
 263                  */
 264                 reader_barrier();
 265         } else {
 266                 _STORE_SHARED(urcu_active_readers, tmp + RCU_GP_COUNT);
 267         }
 268 }
 269
 270 static inline void _rcu_read_unlock(void)
 271 {
 272         long tmp;
 273
 274         tmp = urcu_active_readers;
 275         /*
 276          * Finish using rcu before decrementing the pointer.
 277          * See force_mb_all_threads().
 278          */
 279         if (likely((tmp & RCU_GP_CTR_NEST_MASK) == RCU_GP_COUNT)) {
 280                 reader_barrier();
 281                 _STORE_SHARED(urcu_active_readers,
 282                               urcu_active_readers - RCU_GP_COUNT);
 283                 /* write urcu_active_readers before read futex */
 284                 reader_barrier();
 285                 wake_up_gp();
 286         } else {
 287                 _STORE_SHARED(urcu_active_readers,
 288                               urcu_active_readers - RCU_GP_COUNT);
 289         }
 290 }
 291
 292 /**
 293  * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
 294  * meant to be read by RCU read-side critical sections. Returns the assigned
 295  * value.
 296  *
 297  * Documents which pointers will be dereferenced by RCU read-side critical
 298  * sections and adds the required memory barriers on architectures requiring
 299  * them. It also makes sure the compiler does not reorder code initializing the
 300  * data structure before its publication.
 301  *
 302  * Should match rcu_dereference_pointer().
 303  */
 304
 305 #define _rcu_assign_pointer(p, v)                       \
 306         ({                                              \
 307                 if (!__builtin_constant_p(v) ||         \
 308                     ((v) != NULL))                      \
 309                         wmb();                          \
 310                 STORE_SHARED(p, v);                     \
 311         })
 312
 313 /**
 314  * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
 315  * is as expected by "old". If succeeds, returns the previous pointer to the
 316  * data structure, which can be safely freed after waiting for a quiescent state
 317  * using synchronize_rcu(). If fails (unexpected value), returns old (which
 318  * should not be freed !).
 319  */
 320
 321 #define _rcu_cmpxchg_pointer(p, old, _new)              \
 322         ({                                              \
 323                 if (!__builtin_constant_p(_new) ||      \
 324                     ((_new) != NULL))                   \
 325                         wmb();                          \
 326                 uatomic_cmpxchg(p, old, _new);          \
 327         })
 328
 329 /**
 330  * _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
 331  * pointer to the data structure, which can be safely freed after waiting for a
 332  * quiescent state using synchronize_rcu().
 333  */
 334
 335 #define _rcu_xchg_pointer(p, v)                         \
 336         ({                                              \
 337                 if (!__builtin_constant_p(v) ||         \
 338                     ((v) != NULL))                      \
 339                         wmb();                          \
 340                 uatomic_xchg(p, v);                     \
 341         })
 342
 343 /*
 344  * Exchanges the pointer and waits for quiescent state.
 345  * The pointer returned can be freed.
 346  */
 347 #define _rcu_publish_content(p, v)                      \
 348         ({                                              \
 349                 void *oldptr;                           \
 350                 oldptr = _rcu_xchg_pointer(p, v);       \
 351                 synchronize_rcu();                      \
 352                 oldptr;                                 \
 353         })
 354
 355 #endif /* _URCU_STATIC_H */