urcu.h

   1 #ifndef _URCU_H
   2 #define _URCU_H
   3
   4 /*
   5  * urcu.h
   6  *
   7  * Userspace RCU header
   8  *
   9  * Copyright February 2009 - Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
  10  *
  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  *
  17  * Distributed under GPLv2
  18  */
  19
  20 #include <stdlib.h>
  21 #include <pthread.h>
  22
  23 /* The "volatile" is due to gcc bugs */
  24 #define barrier() __asm__ __volatile__("": : :"memory")
  25
  26 #define likely(x)       __builtin_expect(!!(x), 1)
  27 #define unlikely(x)     __builtin_expect(!!(x), 0)
  28
  29 /*
  30  * Assume the architecture has coherent caches. Blackfin will want this unset.
  31  */
  32 #define CONFIG_HAVE_MEM_COHERENCY 1
  33
  34 /* Assume P4 or newer */
  35 #define CONFIG_HAVE_FENCE 1
  36
  37 /* Assume SMP machine, given we don't have this information */
  38 #define CONFIG_SMP 1
  39
  40
  41 #ifdef CONFIG_HAVE_MEM_COHERENCY
  42 /*
  43  * Caches are coherent, no need to flush them.
  44  */
  45 #define mc()    barrier()
  46 #define rmc()   barrier()
  47 #define wmc()   barrier()
  48 #else
  49 #error "The architecture must create its own cache flush primitives"
  50 #define mc()    arch_cache_flush()
  51 #define rmc()   arch_cache_flush_read()
  52 #define wmc()   arch_cache_flush_write()
  53 #endif
  54
  55
  56 #ifdef CONFIG_HAVE_MEM_COHERENCY
  57
  58 /* x86 32/64 specific */
  59 #ifdef CONFIG_HAVE_FENCE
  60 #define mb()    asm volatile("mfence":::"memory")
  61 #define rmb()   asm volatile("lfence":::"memory")
  62 #define wmb()   asm volatile("sfence"::: "memory")
  63 #else
  64 /*
  65  * Some non-Intel clones support out of order store. wmb() ceases to be a
  66  * nop for these.
  67  */
  68 #define mb()    asm volatile("lock; addl $0,0(%%esp)":::"memory")
  69 #define rmb()   asm volatile("lock; addl $0,0(%%esp)":::"memory")
  70 #define wmb()   asm volatile("lock; addl $0,0(%%esp)"::: "memory")
  71 #endif
  72
  73 #else /* !CONFIG_HAVE_MEM_COHERENCY */
  74
  75 /*
  76  * Without cache coherency, the memory barriers become cache flushes.
  77  */
  78 #define mb()    mc()
  79 #define rmb()   rmc()
  80 #define wmb()   wmc()
  81
  82 #endif /* !CONFIG_HAVE_MEM_COHERENCY */
  83
  84
  85 #ifdef CONFIG_SMP
  86 #define smp_mb()        mb()
  87 #define smp_rmb()       rmb()
  88 #define smp_wmb()       wmb()
  89 #define smp_mc()        mc()
  90 #define smp_rmc()       rmc()
  91 #define smp_wmc()       wmc()
  92 #else
  93 #define smp_mb()        barrier()
  94 #define smp_rmb()       barrier()
  95 #define smp_wmb()       barrier()
  96 #define smp_mc()        barrier()
  97 #define smp_rmc()       barrier()
  98 #define smp_wmc()       barrier()
  99 #endif
 100
 101 /* REP NOP (PAUSE) is a good thing to insert into busy-wait loops. */
 102 static inline void rep_nop(void)
 103 {
 104         asm volatile("rep; nop" ::: "memory");
 105 }
 106
 107 static inline void cpu_relax(void)
 108 {
 109         rep_nop();
 110 }
 111
 112 static inline void atomic_inc(int *v)
 113 {
 114         asm volatile("lock; incl %0"
 115                      : "+m" (*v));
 116 }
 117
 118 #define xchg(ptr, v)                                                    \
 119         ((__typeof__(*(ptr)))__xchg((unsigned long)(v), (ptr), sizeof(*(ptr))))
 120
 121 struct __xchg_dummy {
 122         unsigned long a[100];
 123 };
 124 #define __xg(x) ((struct __xchg_dummy *)(x))
 125
 126 /*
 127  * Note: no "lock" prefix even on SMP: xchg always implies lock anyway
 128  * Note 2: xchg has side effect, so that attribute volatile is necessary,
 129  *        but generally the primitive is invalid, *ptr is output argument. --ANK
 130  * x is considered local, ptr is considered remote.
 131  */
 132 static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
 133                                    int size)
 134 {
 135         switch (size) {
 136         case 1:
 137                 asm volatile("xchgb %b0,%1"
 138                              : "=q" (x)
 139                              : "m" (*__xg(ptr)), "0" (x)
 140                              : "memory");
 141                 break;
 142         case 2:
 143                 asm volatile("xchgw %w0,%1"
 144                              : "=r" (x)
 145                              : "m" (*__xg(ptr)), "0" (x)
 146                              : "memory");
 147                 break;
 148         case 4:
 149                 asm volatile("xchgl %k0,%1"
 150                              : "=r" (x)
 151                              : "m" (*__xg(ptr)), "0" (x)
 152                              : "memory");
 153                 break;
 154         case 8:
 155                 asm volatile("xchgq %0,%1"
 156                              : "=r" (x)
 157                              : "m" (*__xg(ptr)), "0" (x)
 158                              : "memory");
 159                 break;
 160         }
 161         smp_wmc();
 162         return x;
 163 }
 164
 165 /* Nop everywhere except on alpha. */
 166 #define smp_read_barrier_depends()
 167
 168 /*
 169  * Prevent the compiler from merging or refetching accesses.  The compiler
 170  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
 171  * but only when the compiler is aware of some particular ordering.  One way
 172  * to make the compiler aware of ordering is to put the two invocations of
 173  * ACCESS_ONCE() in different C statements.
 174  *
 175  * This macro does absolutely -nothing- to prevent the CPU from reordering,
 176  * merging, or refetching absolutely anything at any time.  Its main intended
 177  * use is to mediate communication between process-level code and irq/NMI
 178  * handlers, all running on the same CPU.
 179  */
 180 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 181
 182 /*
 183  * Load a data from remote memory, doing a cache flush if required.
 184  */
 185 #define LOAD_REMOTE(p)         ({ \
 186                                 smp_rmc(); \
 187                                 typeof(p) _________p1 = ACCESS_ONCE(p); \
 188                                 (_________p1); \
 189                                 })
 190
 191 /*
 192  * Store v into x, where x is located in remote memory. Performs the required
 193  * cache flush after writing.
 194  */
 195 #define STORE_REMOTE(x, v) \
 196         do { \
 197                 (x) = (v); \
 198                 smp_wmc; \
 199         } while (0)
 200
 201 /**
 202  * rcu_dereference - fetch an RCU-protected pointer in an
 203  * RCU read-side critical section.  This pointer may later
 204  * be safely dereferenced.
 205  *
 206  * Inserts memory barriers on architectures that require them
 207  * (currently only the Alpha), and, more importantly, documents
 208  * exactly which pointers are protected by RCU.
 209  */
 210
 211 #define rcu_dereference(p)     ({ \
 212                                 typeof(p) _________p1 = LOAD_REMOTE(p); \
 213                                 smp_read_barrier_depends(); \
 214                                 (_________p1); \
 215                                 })
 216
 217
 218
 219 #define SIGURCU SIGUSR1
 220
 221 /*
 222  * If a reader is really non-cooperative and refuses to commit its
 223  * urcu_active_readers count to memory (there is no barrier in the reader
 224  * per-se), kick it after a few loops waiting for it.
 225  */
 226 #define KICK_READER_LOOPS 10000
 227
 228 #ifdef DEBUG_YIELD
 229 #include <sched.h>
 230 #include <time.h>
 231 #include <pthread.h>
 232 #include <unistd.h>
 233
 234 #define YIELD_READ      (1 << 0)
 235 #define YIELD_WRITE     (1 << 1)
 236
 237 /* Updates without DEBUG_FULL_MB are much slower. Account this in the delay */
 238 #ifdef DEBUG_FULL_MB
 239 /* maximum sleep delay, in us */
 240 #define MAX_SLEEP 50
 241 #else
 242 #define MAX_SLEEP 30000
 243 #endif
 244
 245 extern unsigned int yield_active;
 246 extern unsigned int __thread rand_yield;
 247
 248 static inline void debug_yield_read(void)
 249 {
 250         if (yield_active & YIELD_READ)
 251                 if (rand_r(&rand_yield) & 0x1)
 252                         usleep(rand_r(&rand_yield) % MAX_SLEEP);
 253 }
 254
 255 static inline void debug_yield_write(void)
 256 {
 257         if (yield_active & YIELD_WRITE)
 258                 if (rand_r(&rand_yield) & 0x1)
 259                         usleep(rand_r(&rand_yield) % MAX_SLEEP);
 260 }
 261
 262 static inline void debug_yield_init(void)
 263 {
 264         rand_yield = time(NULL) ^ pthread_self();
 265 }
 266 #else
 267 static inline void debug_yield_read(void)
 268 {
 269 }
 270
 271 static inline void debug_yield_write(void)
 272 {
 273 }
 274
 275 static inline void debug_yield_init(void)
 276 {
 277
 278 }
 279 #endif
 280
 281 #ifdef DEBUG_FULL_MB
 282 static inline void reader_barrier()
 283 {
 284         smp_mb();
 285 }
 286 #else
 287 static inline void reader_barrier()
 288 {
 289         barrier();
 290 }
 291 #endif
 292
 293 /*
 294  * The trick here is that RCU_GP_CTR_BIT must be a multiple of 8 so we can use a
 295  * full 8-bits, 16-bits or 32-bits bitmask for the lower order bits.
 296  */
 297 #define RCU_GP_COUNT            (1UL << 0)
 298 /* Use the amount of bits equal to half of the architecture long size */
 299 #define RCU_GP_CTR_BIT          (1UL << (sizeof(long) << 2))
 300 #define RCU_GP_CTR_NEST_MASK    (RCU_GP_CTR_BIT - 1)
 301
 302 /*
 303  * Global quiescent period counter with low-order bits unused.
 304  * Using a int rather than a char to eliminate false register dependencies
 305  * causing stalls on some architectures.
 306  */
 307 extern long urcu_gp_ctr;
 308
 309 extern long __thread urcu_active_readers;
 310
 311 static inline int rcu_old_gp_ongoing(long *value)
 312 {
 313         long v;
 314
 315         if (value == NULL)
 316                 return 0;
 317         /*
 318          * Make sure both tests below are done on the same version of *value
 319          * to insure consistency.
 320          */
 321         v = LOAD_REMOTE(*value);
 322         return (v & RCU_GP_CTR_NEST_MASK) &&
 323                  ((v ^ urcu_gp_ctr) & RCU_GP_CTR_BIT);
 324 }
 325
 326 static inline void rcu_read_lock(void)
 327 {
 328         long tmp;
 329
 330         tmp = urcu_active_readers;
 331         /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
 332         /*
 333          * The data dependency "read urcu_gp_ctr, write urcu_active_readers",
 334          * serializes those two memory operations. We are not using STORE_REMOTE
 335          * and LOAD_REMOTE here (although we should) because the writer will
 336          * wake us up with a signal which does a flush in its handler to perform
 337          * urcu_gp_ctr re-read and urcu_active_readers commit to main memory.
 338          */
 339         if (likely(!(tmp & RCU_GP_CTR_NEST_MASK)))
 340                 urcu_active_readers = ACCESS_ONCE(urcu_gp_ctr);
 341         else
 342                 urcu_active_readers = tmp + RCU_GP_COUNT;
 343         /*
 344          * Increment active readers count before accessing the pointer.
 345          * See force_mb_all_threads().
 346          */
 347         reader_barrier();
 348 }
 349
 350 static inline void rcu_read_unlock(void)
 351 {
 352         reader_barrier();
 353         /*
 354          * Finish using rcu before decrementing the pointer.
 355          * See force_mb_all_threads().
 356          */
 357         urcu_active_readers -= RCU_GP_COUNT;
 358 }
 359
 360 /**
 361  * rcu_assign_pointer - assign (publicize) a pointer to a newly
 362  * initialized structure that will be dereferenced by RCU read-side
 363  * critical sections.  Returns the value assigned.
 364  *
 365  * Inserts memory barriers on architectures that require them
 366  * (pretty much all of them other than x86), and also prevents
 367  * the compiler from reordering the code that initializes the
 368  * structure after the pointer assignment.  More importantly, this
 369  * call documents which pointers will be dereferenced by RCU read-side
 370  * code.
 371  */
 372
 373 #define rcu_assign_pointer(p, v) \
 374         ({ \
 375                 if (!__builtin_constant_p(v) || \
 376                     ((v) != NULL)) \
 377                         wmb(); \
 378                 (p) = (v); \
 379                 smp_wmc(); \
 380         })
 381
 382 #define rcu_xchg_pointer(p, v) \
 383         ({ \
 384                 if (!__builtin_constant_p(v) || \
 385                     ((v) != NULL)) \
 386                         wmb(); \
 387                 xchg(p, v); \
 388         })
 389
 390 extern void synchronize_rcu(void);
 391
 392 /*
 393  * Exchanges the pointer and waits for quiescent state.
 394  * The pointer returned can be freed.
 395  */
 396 #define urcu_publish_content(p, v) \
 397         ({ \
 398                 void *oldptr; \
 399                 oldptr = rcu_xchg_pointer(p, v); \
 400                 synchronize_rcu(); \
 401                 oldptr; \
 402         })
 403
 404 /*
 405  * Reader thread registration.
 406  */
 407 extern void urcu_register_thread(void);
 408 extern void urcu_unregister_thread(void);
 409
 410 #endif /* _URCU_H */