lib/ovs-thread.h

   1 /*
   2  * Copyright (c) 2013, 2014 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 #ifndef OVS_THREAD_H
  18 #define OVS_THREAD_H 1
  19
  20 #include <pthread.h>
  21 #include <stddef.h>
  22 #include <sys/types.h>
  23 #include "ovs-atomic.h"
  24 #include "util.h"
  25
  26
  27 /* Mutex. */
  28 struct OVS_LOCKABLE ovs_mutex {
  29     pthread_mutex_t lock;
  30     const char *where;
  31 };
  32
  33 /* "struct ovs_mutex" initializer. */
  34 #ifdef PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP
  35 #define OVS_MUTEX_INITIALIZER { PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP, NULL }
  36 #else
  37 #define OVS_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER, NULL }
  38 #endif
  39
  40 #ifdef PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP
  41 #define OVS_ADAPTIVE_MUTEX_INITIALIZER                  \
  42     { PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP, NULL }
  43 #else
  44 #define OVS_ADAPTIVE_MUTEX_INITIALIZER OVS_MUTEX_INITIALIZER
  45 #endif
  46
  47 /* ovs_mutex functions analogous to pthread_mutex_*() functions.
  48  *
  49  * Most of these functions abort the process with an error message on any
  50  * error.  ovs_mutex_trylock() is an exception: it passes through a 0 or EBUSY
  51  * return value to the caller and aborts on any other error. */
  52 void ovs_mutex_init(const struct ovs_mutex *);
  53 void ovs_mutex_init_recursive(const struct ovs_mutex *);
  54 void ovs_mutex_init_adaptive(const struct ovs_mutex *);
  55 void ovs_mutex_destroy(const struct ovs_mutex *);
  56 void ovs_mutex_unlock(const struct ovs_mutex *mutex) OVS_RELEASES(mutex);
  57 void ovs_mutex_lock_at(const struct ovs_mutex *mutex, const char *where)
  58     OVS_ACQUIRES(mutex);
  59 #define ovs_mutex_lock(mutex) \
  60         ovs_mutex_lock_at(mutex, SOURCE_LOCATOR)
  61
  62 int ovs_mutex_trylock_at(const struct ovs_mutex *mutex, const char *where)
  63     OVS_TRY_LOCK(0, mutex);
  64 #define ovs_mutex_trylock(mutex) \
  65         ovs_mutex_trylock_at(mutex, SOURCE_LOCATOR)
  66
  67 void ovs_mutex_cond_wait(pthread_cond_t *, const struct ovs_mutex *);
  68
  69 /* Wrappers for pthread_mutex_*() that abort the process on any error.
  70  * This is still needed when ovs-atomic-pthreads.h is used. */
  71 void xpthread_mutex_lock(pthread_mutex_t *mutex);
  72 void xpthread_mutex_unlock(pthread_mutex_t *mutex);
  73
  74 /* Wrappers for pthread_mutexattr_*() that abort the process on any error. */
  75 void xpthread_mutexattr_init(pthread_mutexattr_t *);
  76 void xpthread_mutexattr_destroy(pthread_mutexattr_t *);
  77 void xpthread_mutexattr_settype(pthread_mutexattr_t *, int type);
  78 void xpthread_mutexattr_gettype(pthread_mutexattr_t *, int *typep);
  79
  80 /* Read-write lock.
  81  *
  82  * An ovs_rwlock does not support recursive readers, because POSIX allows
  83  * taking the reader lock recursively to deadlock when a thread is waiting on
  84  * the write-lock.  (NetBSD does deadlock.)  glibc rwlocks in their default
  85  * configuration do not deadlock, but ovs_rwlock_init() initializes rwlocks as
  86  * non-recursive (which will deadlock) for two reasons:
  87  *
  88  *     - glibc only provides fairness to writers in this mode.
  89  *
  90  *     - It's better to find bugs in the primary Open vSwitch target rather
  91  *       than exposing them only to porters. */
  92 struct OVS_LOCKABLE ovs_rwlock {
  93     pthread_rwlock_t lock;
  94     const char *where;
  95 };
  96
  97 /* Initializer. */
  98 #ifdef PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP
  99 #define OVS_RWLOCK_INITIALIZER \
 100         { PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP, NULL }
 101 #else
 102 #define OVS_RWLOCK_INITIALIZER { PTHREAD_RWLOCK_INITIALIZER, NULL }
 103 #endif
 104
 105 /* ovs_rwlock functions analogous to pthread_rwlock_*() functions.
 106  *
 107  * Most of these functions abort the process with an error message on any
 108  * error.  The "trylock" functions are exception: they pass through a 0 or
 109  * EBUSY return value to the caller and abort on any other error. */
 110 void ovs_rwlock_init(const struct ovs_rwlock *);
 111 void ovs_rwlock_destroy(const struct ovs_rwlock *);
 112 void ovs_rwlock_unlock(const struct ovs_rwlock *rwlock) OVS_RELEASES(rwlock);
 113
 114 /* Wrappers for pthread_rwlockattr_*() that abort the process on any error. */
 115 void xpthread_rwlockattr_init(pthread_rwlockattr_t *);
 116 void xpthread_rwlockattr_destroy(pthread_rwlockattr_t *);
 117 #ifdef PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP
 118 void xpthread_rwlockattr_setkind_np(pthread_rwlockattr_t *, int kind);
 119 #endif
 120
 121 void ovs_rwlock_wrlock_at(const struct ovs_rwlock *rwlock, const char *where)
 122     OVS_ACQ_WRLOCK(rwlock);
 123 #define ovs_rwlock_wrlock(rwlock) \
 124         ovs_rwlock_wrlock_at(rwlock, SOURCE_LOCATOR)
 125
 126 int ovs_rwlock_trywrlock_at(const struct ovs_rwlock *rwlock, const char *where)
 127     OVS_TRY_WRLOCK(0, rwlock);
 128 #define ovs_rwlock_trywrlock(rwlock) \
 129     ovs_rwlock_trywrlock_at(rwlock, SOURCE_LOCATOR)
 130
 131 void ovs_rwlock_rdlock_at(const struct ovs_rwlock *rwlock, const char *where)
 132     OVS_ACQ_RDLOCK(rwlock);
 133 #define ovs_rwlock_rdlock(rwlock) \
 134         ovs_rwlock_rdlock_at(rwlock, SOURCE_LOCATOR)
 135
 136 int ovs_rwlock_tryrdlock_at(const struct ovs_rwlock *rwlock, const char *where)
 137     OVS_TRY_RDLOCK(0, rwlock);
 138 #define ovs_rwlock_tryrdlock(rwlock) \
 139         ovs_rwlock_tryrdlock_at(rwlock, SOURCE_LOCATOR)
 140
 141 /* Wrappers for xpthread_cond_*() that abort the process on any error.
 142  *
 143  * Use ovs_mutex_cond_wait() to wait for a condition. */
 144 void xpthread_cond_init(pthread_cond_t *, pthread_condattr_t *);
 145 void xpthread_cond_destroy(pthread_cond_t *);
 146 void xpthread_cond_signal(pthread_cond_t *);
 147 void xpthread_cond_broadcast(pthread_cond_t *);
 148
 149 void xpthread_key_create(pthread_key_t *, void (*destructor)(void *));
 150 void xpthread_key_delete(pthread_key_t);
 151 void xpthread_setspecific(pthread_key_t, const void *);
 152
 153 void xpthread_create(pthread_t *, pthread_attr_t *, void *(*)(void *), void *);
 154 void xpthread_join(pthread_t, void **);
 155 \f
 156 /* Per-thread data.
 157  *
 158  *
 159  * Standard Forms
 160  * ==============
 161  *
 162  * Multiple forms of standard per-thread data exist, each with its own pluses
 163  * and minuses.  In general, if one of these forms is appropriate, then it's a
 164  * good idea to use it:
 165  *
 166  *     - POSIX per-thread data via pthread_key_t is portable to any pthreads
 167  *       implementation, and allows a destructor function to be defined.  It
 168  *       only (directly) supports per-thread pointers, which are always
 169  *       initialized to NULL.  It requires once-only allocation of a
 170  *       pthread_key_t value.  It is relatively slow.  Typically few
 171  *       "pthread_key_t"s are available (POSIX requires only at least 128,
 172  *       glibc supplies only 1024).
 173  *
 174  *     - The thread_local feature newly defined in C11 <threads.h> works with
 175  *       any data type and initializer, and it is fast.  thread_local does not
 176  *       require once-only initialization like pthread_key_t.  C11 does not
 177  *       define what happens if one attempts to access a thread_local object
 178  *       from a thread other than the one to which that object belongs.  There
 179  *       is no provision to call a user-specified destructor when a thread
 180  *       ends.  Typical implementations allow for an arbitrary amount of
 181  *       thread_local storage, but statically allocated only.
 182  *
 183  *     - The __thread keyword is a GCC extension similar to thread_local but
 184  *       with a longer history.  __thread is not portable to every GCC version
 185  *       or environment.  __thread does not restrict the use of a thread-local
 186  *       object outside its own thread.
 187  *
 188  * Here's a handy summary:
 189  *
 190  *                     pthread_key_t     thread_local       __thread
 191  *                     -------------     ------------     -------------
 192  * portability             high               low             medium
 193  * speed                    low              high               high
 194  * supports destructors?    yes                no                 no
 195  * needs key allocation?    yes                no                 no
 196  * arbitrary initializer?    no               yes                yes
 197  * cross-thread access?     yes                no                yes
 198  * amount available?        few            arbitrary         arbitrary
 199  * dynamically allocated?   yes                no                 no
 200  *
 201  *
 202  * Extensions
 203  * ==========
 204  *
 205  * OVS provides some extensions and wrappers:
 206  *
 207  *     - In a situation where the performance of thread_local or __thread is
 208  *       desirable, but portability is required, DEFINE_STATIC_PER_THREAD_DATA
 209  *       and DECLARE_EXTERN_PER_THREAD_DATA/DEFINE_EXTERN_PER_THREAD_DATA may
 210  *       be appropriate (see below).
 211  *
 212  *     - DEFINE_PER_THREAD_MALLOCED_DATA can be convenient for simple
 213  *       per-thread malloc()'d buffers.
 214  *
 215  *     - struct ovs_tsd provides an alternative to pthread_key_t that isn't
 216  *       limited to a small number of keys.
 217  */
 218
 219 /* For static data, use this macro in a source file:
 220  *
 221  *    DEFINE_STATIC_PER_THREAD_DATA(TYPE, NAME, INITIALIZER).
 222  *
 223  * For global data, "declare" the data in the header and "define" it in
 224  * the source file, with:
 225  *
 226  *    DECLARE_EXTERN_PER_THREAD_DATA(TYPE, NAME).
 227  *    DEFINE_EXTERN_PER_THREAD_DATA(NAME, INITIALIZER).
 228  *
 229  * One should prefer to use POSIX per-thread data, via pthread_key_t, when its
 230  * performance is acceptable, because of its portability (see the table above).
 231  * This macro is an alternatives that takes advantage of thread_local (and
 232  * __thread), for its performance, when it is available, and falls back to
 233  * POSIX per-thread data otherwise.
 234  *
 235  * Defines per-thread variable NAME with the given TYPE, initialized to
 236  * INITIALIZER (which must be valid as an initializer for a variable with
 237  * static lifetime).
 238  *
 239  * The public interface to the variable is:
 240  *
 241  *    TYPE *NAME_get(void)
 242  *    TYPE *NAME_get_unsafe(void)
 243  *
 244  *       Returns the address of this thread's instance of NAME.
 245  *
 246  *       Use NAME_get() in a context where this might be the first use of the
 247  *       per-thread variable in the program.  Use NAME_get_unsafe(), which
 248  *       avoids a conditional test and is thus slightly faster, in a context
 249  *       where one knows that NAME_get() has already been called previously.
 250  *
 251  * There is no "NAME_set()" (or "NAME_set_unsafe()") function.  To set the
 252  * value of the per-thread variable, dereference the pointer returned by
 253  * TYPE_get() or TYPE_get_unsafe(), e.g. *TYPE_get() = 0.
 254  */
 255 #if HAVE_THREAD_LOCAL || HAVE___THREAD
 256
 257 #if HAVE_THREAD_LOCAL
 258 #include <threads.h>
 259 #elif HAVE___THREAD
 260 #define thread_local __thread
 261 #else
 262 #error
 263 #endif
 264
 265 #define DEFINE_STATIC_PER_THREAD_DATA(TYPE, NAME, ...)                  \
 266     typedef TYPE NAME##_type;                                           \
 267                                                                         \
 268     static NAME##_type *                                                \
 269     NAME##_get_unsafe(void)                                             \
 270     {                                                                   \
 271         static thread_local NAME##_type var = __VA_ARGS__;              \
 272         return &var;                                                    \
 273     }                                                                   \
 274                                                                         \
 275     static NAME##_type *                                                \
 276     NAME##_get(void)                                                    \
 277     {                                                                   \
 278         return NAME##_get_unsafe();                                     \
 279     }
 280 #define DECLARE_EXTERN_PER_THREAD_DATA(TYPE, NAME)                      \
 281     typedef TYPE NAME##_type;                                           \
 282     extern thread_local NAME##_type NAME##_var;                         \
 283                                                                         \
 284     static inline NAME##_type *                                         \
 285     NAME##_get_unsafe(void)                                             \
 286     {                                                                   \
 287         return &NAME##_var;                                             \
 288     }                                                                   \
 289                                                                         \
 290     static inline NAME##_type *                                         \
 291     NAME##_get(void)                                                    \
 292     {                                                                   \
 293         return NAME##_get_unsafe();                                     \
 294     }
 295 #define DEFINE_EXTERN_PER_THREAD_DATA(NAME, ...)         \
 296     thread_local NAME##_type NAME##_var = __VA_ARGS__;
 297 #else  /* no C implementation support for thread-local storage  */
 298 #define DEFINE_STATIC_PER_THREAD_DATA(TYPE, NAME, ...)                  \
 299     typedef TYPE NAME##_type;                                           \
 300     static pthread_key_t NAME##_key;                                    \
 301                                                                         \
 302     static NAME##_type *                                                \
 303     NAME##_get_unsafe(void)                                             \
 304     {                                                                   \
 305         return pthread_getspecific(NAME##_key);                         \
 306     }                                                                   \
 307                                                                         \
 308     static void                                                         \
 309     NAME##_once_init(void)                                              \
 310     {                                                                   \
 311         if (pthread_key_create(&NAME##_key, free)) {                    \
 312             abort();                                                    \
 313         }                                                               \
 314     }                                                                   \
 315                                                                         \
 316     static NAME##_type *                                                \
 317     NAME##_get(void)                                                    \
 318     {                                                                   \
 319         static pthread_once_t once = PTHREAD_ONCE_INIT;                 \
 320         NAME##_type *value;                                             \
 321                                                                         \
 322         pthread_once(&once, NAME##_once_init);                          \
 323         value = NAME##_get_unsafe();                                    \
 324         if (!value) {                                                   \
 325             static const NAME##_type initial_value = __VA_ARGS__;       \
 326                                                                         \
 327             value = malloc(sizeof *value);                              \
 328             if (value == NULL) {                                        \
 329                 out_of_memory();                                        \
 330             }                                                           \
 331             *value = initial_value;                                     \
 332             xpthread_setspecific(NAME##_key, value);                    \
 333         }                                                               \
 334         return value;                                                   \
 335     }
 336 #define DECLARE_EXTERN_PER_THREAD_DATA(TYPE, NAME)                      \
 337     typedef TYPE NAME##_type;                                           \
 338     static pthread_key_t NAME##_key;                                    \
 339                                                                         \
 340     static inline NAME##_type *                                         \
 341     NAME##_get_unsafe(void)                                             \
 342     {                                                                   \
 343         return pthread_getspecific(NAME##_key);                         \
 344     }                                                                   \
 345                                                                         \
 346     NAME##_type *NAME##_get(void);
 347 #define DEFINE_EXTERN_PER_THREAD_DATA(NAME, ...)                        \
 348     static void                                                         \
 349     NAME##_once_init(void)                                              \
 350     {                                                                   \
 351         if (pthread_key_create(&NAME##_key, free)) {                    \
 352             abort();                                                    \
 353         }                                                               \
 354     }                                                                   \
 355                                                                         \
 356     NAME##_type *                                                       \
 357     NAME##_get(void)                                                    \
 358     {                                                                   \
 359         static pthread_once_t once = PTHREAD_ONCE_INIT;                 \
 360         NAME##_type *value;                                             \
 361                                                                         \
 362         pthread_once(&once, NAME##_once_init);                          \
 363         value = NAME##_get_unsafe();                                    \
 364         if (!value) {                                                   \
 365             static const NAME##_type initial_value = __VA_ARGS__;       \
 366                                                                         \
 367             value = malloc(sizeof *value);                              \
 368             if (value == NULL) {                                        \
 369                 out_of_memory();                                        \
 370             }                                                           \
 371             *value = initial_value;                                     \
 372             xpthread_setspecific(NAME##_key, value);                    \
 373         }                                                               \
 374         return value;                                                   \
 375     }
 376 #endif
 377
 378 /* DEFINE_PER_THREAD_MALLOCED_DATA(TYPE, NAME).
 379  *
 380  * This is a simple wrapper around POSIX per-thread data primitives.  It
 381  * defines per-thread variable NAME with the given TYPE, which must be a
 382  * pointer type.  In each thread, the per-thread variable is initialized to
 383  * NULL.  When a thread terminates, the variable is freed with free().
 384  *
 385  * The public interface to the variable is:
 386  *
 387  *    TYPE NAME_get(void)
 388  *    TYPE NAME_get_unsafe(void)
 389  *
 390  *       Returns the value of per-thread variable NAME in this thread.
 391  *
 392  *       Use NAME_get() in a context where this might be the first use of the
 393  *       per-thread variable in the program.  Use NAME_get_unsafe(), which
 394  *       avoids a conditional test and is thus slightly faster, in a context
 395  *       where one knows that NAME_get() has already been called previously.
 396  *
 397  *    TYPE NAME_set(TYPE new_value)
 398  *    TYPE NAME_set_unsafe(TYPE new_value)
 399  *
 400  *       Sets the value of per-thread variable NAME to 'new_value' in this
 401  *       thread, and returns its previous value.
 402  *
 403  *       Use NAME_set() in a context where this might be the first use of the
 404  *       per-thread variable in the program.  Use NAME_set_unsafe(), which
 405  *       avoids a conditional test and is thus slightly faster, in a context
 406  *       where one knows that NAME_set() has already been called previously.
 407  */
 408 #define DEFINE_PER_THREAD_MALLOCED_DATA(TYPE, NAME)     \
 409     static pthread_key_t NAME##_key;                    \
 410                                                         \
 411     static void                                         \
 412     NAME##_once_init(void)                              \
 413     {                                                   \
 414         if (pthread_key_create(&NAME##_key, free)) {    \
 415             abort();                                    \
 416         }                                               \
 417     }                                                   \
 418                                                         \
 419     static void                                         \
 420     NAME##_init(void)                                   \
 421     {                                                   \
 422         static pthread_once_t once = PTHREAD_ONCE_INIT; \
 423         pthread_once(&once, NAME##_once_init);          \
 424     }                                                   \
 425                                                         \
 426     static TYPE                                         \
 427     NAME##_get_unsafe(void)                             \
 428     {                                                   \
 429         return pthread_getspecific(NAME##_key);         \
 430     }                                                   \
 431                                                         \
 432     static OVS_UNUSED TYPE                              \
 433     NAME##_get(void)                                    \
 434     {                                                   \
 435         NAME##_init();                                  \
 436         return NAME##_get_unsafe();                     \
 437     }                                                   \
 438                                                         \
 439     static TYPE                                         \
 440     NAME##_set_unsafe(TYPE value)                       \
 441     {                                                   \
 442         TYPE old_value = NAME##_get_unsafe();           \
 443         xpthread_setspecific(NAME##_key, value);        \
 444         return old_value;                               \
 445     }                                                   \
 446                                                         \
 447     static OVS_UNUSED TYPE                              \
 448     NAME##_set(TYPE value)                              \
 449     {                                                   \
 450         NAME##_init();                                  \
 451         return NAME##_set_unsafe(value);                \
 452     }
 453
 454 /* Dynamically allocated thread-specific data with lots of slots.
 455  *
 456  * pthread_key_t can provide as few as 128 pieces of thread-specific data (even
 457  * glibc is limited to 1,024).  Thus, one must be careful to allocate only a
 458  * few keys globally.  One cannot, for example, allocate a key for every
 459  * instance of a data structure if there might be an arbitrary number of those
 460  * data structures.
 461  *
 462  * This API is similar to the pthread one (simply search and replace pthread_
 463  * by ovsthread_) but it a much larger limit that can be raised if necessary
 464  * (by recompiling).  Thus, one may more freely use this form of
 465  * thread-specific data.
 466  *
 467  * ovsthread_key_t also differs from pthread_key_t in the following ways:
 468  *
 469  *    - Destructors must not access thread-specific data (via ovsthread_key).
 470  *
 471  *    - The pthread_key_t API allows concurrently exiting threads to start
 472  *      executing the destructor after pthread_key_delete() returns.  The
 473  *      ovsthread_key_t API guarantees that, when ovsthread_key_delete()
 474  *      returns, all destructors have returned and no new ones will start
 475  *      execution.
 476  */
 477 typedef struct ovsthread_key *ovsthread_key_t;
 478
 479 void ovsthread_key_create(ovsthread_key_t *, void (*destructor)(void *));
 480 void ovsthread_key_delete(ovsthread_key_t);
 481
 482 void ovsthread_setspecific(ovsthread_key_t, const void *);
 483 void *ovsthread_getspecific(ovsthread_key_t);
 484 \f
 485 /* Convenient once-only execution.
 486  *
 487  *
 488  * Problem
 489  * =======
 490  *
 491  * POSIX provides pthread_once_t and pthread_once() as primitives for running a
 492  * set of code only once per process execution.  They are used like this:
 493  *
 494  *     static void run_once(void) { ...initialization... }
 495  *     static pthread_once_t once = PTHREAD_ONCE_INIT;
 496  * ...
 497  *     pthread_once(&once, run_once);
 498  *
 499  * pthread_once() does not allow passing any parameters to the initialization
 500  * function, which is often inconvenient, because it means that the function
 501  * can only access data declared at file scope.
 502  *
 503  *
 504  * Solution
 505  * ========
 506  *
 507  * Use ovsthread_once, like this, instead:
 508  *
 509  *     static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER;
 510  *
 511  *     if (ovsthread_once_start(&once)) {
 512  *         ...initialization...
 513  *         ovsthread_once_done(&once);
 514  *     }
 515  */
 516
 517 struct ovsthread_once {
 518     atomic_bool done;
 519     struct ovs_mutex mutex;
 520 };
 521
 522 #define OVSTHREAD_ONCE_INITIALIZER              \
 523     {                                           \
 524         ATOMIC_VAR_INIT(false),                 \
 525         OVS_MUTEX_INITIALIZER,                  \
 526     }
 527
 528 static inline bool ovsthread_once_start(struct ovsthread_once *once)
 529     OVS_TRY_LOCK(true, once->mutex);
 530 void ovsthread_once_done(struct ovsthread_once *once)
 531     OVS_RELEASES(once->mutex);
 532
 533 bool ovsthread_once_start__(struct ovsthread_once *once)
 534     OVS_TRY_LOCK(false, once->mutex);
 535
 536 static inline bool
 537 ovsthread_once_is_done__(struct ovsthread_once *once)
 538 {
 539     bool done;
 540
 541     atomic_read_explicit(&once->done, &done, memory_order_relaxed);
 542     return done;
 543 }
 544
 545 /* Returns true if this is the first call to ovsthread_once_start() for
 546  * 'once'.  In this case, the caller should perform whatever initialization
 547  * actions it needs to do, then call ovsthread_once_done() for 'once'.
 548  *
 549  * Returns false if this is not the first call to ovsthread_once_start() for
 550  * 'once'.  In this case, the call will not return until after
 551  * ovsthread_once_done() has been called. */
 552 static inline bool
 553 ovsthread_once_start(struct ovsthread_once *once)
 554 {
 555     return OVS_UNLIKELY(!ovsthread_once_is_done__(once)
 556                         && !ovsthread_once_start__(once));
 557 }
 558 \f
 559 /* Thread ID.
 560  *
 561  * pthread_t isn't so nice for some purposes.  Its size and representation are
 562  * implementation dependent, which means that there is no way to hash it.
 563  * This thread ID avoids the problem.
 564  */
 565
 566 DECLARE_EXTERN_PER_THREAD_DATA(unsigned int, ovsthread_id);
 567
 568 /* Returns a per-thread identifier unique within the lifetime of the
 569  * process. */
 570 static inline unsigned int
 571 ovsthread_id_self(void)
 572 {
 573     return *ovsthread_id_get();
 574 }
 575 \f
 576 /* Simulated global counter.
 577  *
 578  * Incrementing such a counter is meant to be cheaper than incrementing a
 579  * global counter protected by a lock.  It is probably more expensive than
 580  * incrementing a truly thread-local variable, but such a variable has no
 581  * straightforward way to get the sum.
 582  *
 583  *
 584  * Thread-safety
 585  * =============
 586  *
 587  * Fully thread-safe. */
 588
 589 struct ovsthread_counter *ovsthread_counter_create(void);
 590 void ovsthread_counter_destroy(struct ovsthread_counter *);
 591 void ovsthread_counter_inc(struct ovsthread_counter *, unsigned long long int);
 592 unsigned long long int ovsthread_counter_read(
 593     const struct ovsthread_counter *);
 594 \f
 595 void assert_single_threaded_at(const char *where);
 596 #define assert_single_threaded() assert_single_threaded_at(SOURCE_LOCATOR)
 597
 598 pid_t xfork_at(const char *where);
 599 #define xfork() xfork_at(SOURCE_LOCATOR)
 600
 601 void forbid_forking(const char *reason);
 602 bool may_fork(void);
 603 \f
 604 /* Useful functions related to threading. */
 605
 606 int count_cpu_cores(void);
 607
 608 #endif /* ovs-thread.h */