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 <sched.h>
  35
  36 #include <compiler.h>
  37 #include <arch.h>
  38
  39 /*
  40  * Identify a shared load. A smp_rmc() or smp_mc() should come before the load.
  41  */
  42 #define _LOAD_SHARED(p)        ACCESS_ONCE(p)
  43
  44 /*
  45  * Load a data from shared memory, doing a cache flush if required.
  46  */
  47 #define LOAD_SHARED(p)                  \
  48         ({                              \
  49                 smp_rmc();              \
  50                 _LOAD_SHARED(p);        \
  51         })
  52
  53 /*
  54  * Identify a shared store. A smp_wmc() or smp_mc() should follow the store.
  55  */
  56 #define _STORE_SHARED(x, v)     ({ ACCESS_ONCE(x) = (v); })
  57
  58 /*
  59  * Store v into x, where x is located in shared memory. Performs the required
  60  * cache flush after writing. Returns v.
  61  */
  62 #define STORE_SHARED(x, v)              \
  63         ({                              \
  64                 _STORE_SHARED(x, v);    \
  65                 smp_wmc();              \
  66                 (v);                    \
  67         })
  68
  69 /**
  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.
  73  *
  74  * This ensures that the pointer copy is invariant thorough the whole critical
  75  * section.
  76  *
  77  * Inserts memory barriers on architectures that require them (currently only
  78  * Alpha) and documents which pointers are protected by RCU.
  79  *
  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.
  88  *
  89  * Should match rcu_assign_pointer() or rcu_xchg_pointer().
  90  */
  91
  92 #define _rcu_dereference(p)     ({                                      \
  93                                 typeof(p) _________p1 = LOAD_SHARED(p); \
  94                                 smp_read_barrier_depends();             \
  95                                 (_________p1);                          \
  96                                 })
  97
  98 /*
  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.
 104  */
 105
 106 /*
 107  * The signal number used by the RCU library can be overridden with
 108  * -DSIGURCU= when compiling the library.
 109  */
 110 #ifndef SIGURCU
 111 #define SIGURCU SIGUSR1
 112 #endif
 113
 114 /*
 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.
 118  */
 119 #define KICK_READER_LOOPS 10000
 120
 121 /*
 122  * Active attempts to check for reader Q.S. before calling sched_yield().
 123  */
 124 #define RCU_QS_ACTIVE_ATTEMPTS 100
 125
 126 #ifdef DEBUG_RCU
 127 #define rcu_assert(args...)     assert(args)
 128 #else
 129 #define rcu_assert(args...)
 130 #endif
 131
 132 #ifdef DEBUG_YIELD
 133 #include <sched.h>
 134 #include <time.h>
 135 #include <pthread.h>
 136 #include <unistd.h>
 137
 138 #define YIELD_READ      (1 << 0)
 139 #define YIELD_WRITE     (1 << 1)
 140
 141 /*
 142  * Updates without URCU_MB are much slower. Account this in
 143  * the delay.
 144  */
 145 #ifdef URCU_MB
 146 /* maximum sleep delay, in us */
 147 #define MAX_SLEEP 50
 148 #else
 149 #define MAX_SLEEP 30000
 150 #endif
 151
 152 extern unsigned int yield_active;
 153 extern unsigned int __thread rand_yield;
 154
 155 static inline void debug_yield_read(void)
 156 {
 157         if (yield_active & YIELD_READ)
 158                 if (rand_r(&rand_yield) & 0x1)
 159                         usleep(rand_r(&rand_yield) % MAX_SLEEP);
 160 }
 161
 162 static inline void debug_yield_write(void)
 163 {
 164         if (yield_active & YIELD_WRITE)
 165                 if (rand_r(&rand_yield) & 0x1)
 166                         usleep(rand_r(&rand_yield) % MAX_SLEEP);
 167 }
 168
 169 static inline void debug_yield_init(void)
 170 {
 171         rand_yield = time(NULL) ^ pthread_self();
 172 }
 173 #else
 174 static inline void debug_yield_read(void)
 175 {
 176 }
 177
 178 static inline void debug_yield_write(void)
 179 {
 180 }
 181
 182 static inline void debug_yield_init(void)
 183 {
 184
 185 }
 186 #endif
 187
 188 #ifdef URCU_MB
 189 static inline void reader_barrier()
 190 {
 191         smp_mb();
 192 }
 193 #else
 194 static inline void reader_barrier()
 195 {
 196         barrier();
 197 }
 198 #endif
 199
 200 /*
 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.
 203  */
 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)
 209
 210 /*
 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.
 214  */
 215 extern long urcu_gp_ctr;
 216
 217 struct urcu_reader_status {
 218         long active_readers;
 219         long gp_waiting;
 220 };
 221
 222 extern struct urcu_reader_status __thread urcu_reader_status;
 223
 224 static inline int rcu_old_gp_ongoing(long *value)
 225 {
 226         long v;
 227
 228         if (value == NULL)
 229                 return 0;
 230         /*
 231          * Make sure both tests below are done on the same version of *value
 232          * to insure consistency.
 233          */
 234         v = LOAD_SHARED(*value);
 235         return (v & RCU_GP_CTR_NEST_MASK) &&
 236                  ((v ^ urcu_gp_ctr) & RCU_GP_CTR_BIT);
 237 }
 238
 239 static inline void _rcu_read_lock(void)
 240 {
 241         long tmp, gp_ctr;
 242
 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))) {
 246                 /*
 247                  * volatile accesses can be reordered and optimized when within
 248                  * the same statement.
 249                  */
 250                 if (unlikely((gp_ctr = _LOAD_SHARED(urcu_gp_ctr))
 251                                 & RCU_GP_ONGOING) &&
 252                     unlikely(LOAD_SHARED(urcu_reader_status.gp_waiting))) {
 253                         sched_yield();
 254                         gp_ctr = _LOAD_SHARED(urcu_gp_ctr);
 255                 }
 256                 _STORE_SHARED(urcu_reader_status.active_readers, gp_ctr);
 257                 /*
 258                  * Set active readers count for outermost nesting level before
 259                  * accessing the pointer. See force_mb_all_threads().
 260                  */
 261                 reader_barrier();
 262         } else {
 263                 _STORE_SHARED(urcu_reader_status.active_readers,
 264                               tmp + RCU_GP_COUNT);
 265         }
 266 }
 267
 268 static inline void _rcu_read_unlock(void)
 269 {
 270         /*
 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
 275          * (no nesting).
 276          */
 277         reader_barrier();
 278         _STORE_SHARED(urcu_reader_status.active_readers,
 279                       urcu_reader_status.active_readers - RCU_GP_COUNT);
 280 }
 281
 282 /**
 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
 285  * value.
 286  *
 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.
 291  *
 292  * Should match rcu_dereference_pointer().
 293  */
 294
 295 #define _rcu_assign_pointer(p, v)                       \
 296         ({                                              \
 297                 if (!__builtin_constant_p(v) ||         \
 298                     ((v) != NULL))                      \
 299                         wmb();                          \
 300                 STORE_SHARED(p, v);                     \
 301         })
 302
 303 /**
 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 !).
 309  */
 310
 311 #define _rcu_cmpxchg_pointer(p, old, _new)              \
 312         ({                                              \
 313                 if (!__builtin_constant_p(_new) ||      \
 314                     ((_new) != NULL))                   \
 315                         wmb();                          \
 316                 cmpxchg(p, old, _new);                  \
 317         })
 318
 319 /**
 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().
 323  */
 324
 325 #define _rcu_xchg_pointer(p, v)                         \
 326         ({                                              \
 327                 if (!__builtin_constant_p(v) ||         \
 328                     ((v) != NULL))                      \
 329                         wmb();                          \
 330                 xchg(p, v);                             \
 331         })
 332
 333 /*
 334  * Exchanges the pointer and waits for quiescent state.
 335  * The pointer returned can be freed.
 336  */
 337 #define _rcu_publish_content(p, v)                      \
 338         ({                                              \
 339                 void *oldptr;                           \
 340                 oldptr = _rcu_xchg_pointer(p, v);       \
 341                 synchronize_rcu();                      \
 342                 oldptr;                                 \
 343         })
 344
 345 #endif /* _URCU_STATIC_H */