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;          /* NULL if and only if uninitialized. */
  31 };
  32
  33 /* "struct ovs_mutex" initializer. */
  34 #ifdef PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP
  35 #define OVS_MUTEX_INITIALIZER { PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP, \
  36                                 "<unlocked>" }
  37 #else
  38 #define OVS_MUTEX_INITIALIZER { PTHREAD_MUTEX_INITIALIZER, "<unlocked>" }
  39 #endif
  40
  41 #ifdef PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP
  42 #define OVS_ADAPTIVE_MUTEX_INITIALIZER                  \
  43     { PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP, NULL }
  44 #else
  45 #define OVS_ADAPTIVE_MUTEX_INITIALIZER OVS_MUTEX_INITIALIZER
  46 #endif
  47
  48 /* ovs_mutex functions analogous to pthread_mutex_*() functions.
  49  *
  50  * Most of these functions abort the process with an error message on any
  51  * error.  ovs_mutex_trylock() is an exception: it passes through a 0 or EBUSY
  52  * return value to the caller and aborts on any other error. */
  53 void ovs_mutex_init(const struct ovs_mutex *);
  54 void ovs_mutex_init_recursive(const struct ovs_mutex *);
  55 void ovs_mutex_init_adaptive(const struct ovs_mutex *);
  56 void ovs_mutex_destroy(const struct ovs_mutex *);
  57 void ovs_mutex_unlock(const struct ovs_mutex *mutex) OVS_RELEASES(mutex);
  58 void ovs_mutex_lock_at(const struct ovs_mutex *mutex, const char *where)
  59     OVS_ACQUIRES(mutex);
  60 #define ovs_mutex_lock(mutex) \
  61         ovs_mutex_lock_at(mutex, SOURCE_LOCATOR)
  62
  63 int ovs_mutex_trylock_at(const struct ovs_mutex *mutex, const char *where)
  64     OVS_TRY_LOCK(0, mutex);
  65 #define ovs_mutex_trylock(mutex) \
  66         ovs_mutex_trylock_at(mutex, SOURCE_LOCATOR)
  67
  68 void ovs_mutex_cond_wait(pthread_cond_t *, const struct ovs_mutex *);
  69
  70 /* Wrappers for pthread_mutex_*() that abort the process on any error.
  71  * This is still needed when ovs-atomic-pthreads.h is used. */
  72 void xpthread_mutex_lock(pthread_mutex_t *mutex);
  73 void xpthread_mutex_unlock(pthread_mutex_t *mutex);
  74
  75 /* Wrappers for pthread_mutexattr_*() that abort the process on any error. */
  76 void xpthread_mutexattr_init(pthread_mutexattr_t *);
  77 void xpthread_mutexattr_destroy(pthread_mutexattr_t *);
  78 void xpthread_mutexattr_settype(pthread_mutexattr_t *, int type);
  79 void xpthread_mutexattr_gettype(pthread_mutexattr_t *, int *typep);
  80
  81 /* Read-write lock.
  82  *
  83  * An ovs_rwlock does not support recursive readers, because POSIX allows
  84  * taking the reader lock recursively to deadlock when a thread is waiting on
  85  * the write-lock.  (NetBSD does deadlock.)  glibc rwlocks in their default
  86  * configuration do not deadlock, but ovs_rwlock_init() initializes rwlocks as
  87  * non-recursive (which will deadlock) for two reasons:
  88  *
  89  *     - glibc only provides fairness to writers in this mode.
  90  *
  91  *     - It's better to find bugs in the primary Open vSwitch target rather
  92  *       than exposing them only to porters. */
  93 struct OVS_LOCKABLE ovs_rwlock {
  94     pthread_rwlock_t lock;
  95     const char *where;          /* NULL if and only if uninitialized. */
  96 };
  97
  98 /* Initializer. */
  99 #ifdef PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP
 100 #define OVS_RWLOCK_INITIALIZER \
 101         { PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP, "<unlocked>" }
 102 #else
 103 #define OVS_RWLOCK_INITIALIZER { PTHREAD_RWLOCK_INITIALIZER, "<unlocked>" }
 104 #endif
 105
 106 /* ovs_rwlock functions analogous to pthread_rwlock_*() functions.
 107  *
 108  * Most of these functions abort the process with an error message on any
 109  * error.  The "trylock" functions are exception: they pass through a 0 or
 110  * EBUSY return value to the caller and abort on any other error. */
 111 void ovs_rwlock_init(const struct ovs_rwlock *);
 112 void ovs_rwlock_destroy(const struct ovs_rwlock *);
 113 void ovs_rwlock_unlock(const struct ovs_rwlock *rwlock) OVS_RELEASES(rwlock);
 114
 115 /* Wrappers for pthread_rwlockattr_*() that abort the process on any error. */
 116 void xpthread_rwlockattr_init(pthread_rwlockattr_t *);
 117 void xpthread_rwlockattr_destroy(pthread_rwlockattr_t *);
 118 #ifdef PTHREAD_RWLOCK_WRITER_NONRECURSIVE_INITIALIZER_NP
 119 void xpthread_rwlockattr_setkind_np(pthread_rwlockattr_t *, int kind);
 120 #endif
 121
 122 void ovs_rwlock_wrlock_at(const struct ovs_rwlock *rwlock, const char *where)
 123     OVS_ACQ_WRLOCK(rwlock);
 124 #define ovs_rwlock_wrlock(rwlock) \
 125         ovs_rwlock_wrlock_at(rwlock, SOURCE_LOCATOR)
 126
 127 int ovs_rwlock_trywrlock_at(const struct ovs_rwlock *rwlock, const char *where)
 128     OVS_TRY_WRLOCK(0, rwlock);
 129 #define ovs_rwlock_trywrlock(rwlock) \
 130     ovs_rwlock_trywrlock_at(rwlock, SOURCE_LOCATOR)
 131
 132 void ovs_rwlock_rdlock_at(const struct ovs_rwlock *rwlock, const char *where)
 133     OVS_ACQ_RDLOCK(rwlock);
 134 #define ovs_rwlock_rdlock(rwlock) \
 135         ovs_rwlock_rdlock_at(rwlock, SOURCE_LOCATOR)
 136
 137 int ovs_rwlock_tryrdlock_at(const struct ovs_rwlock *rwlock, const char *where)
 138     OVS_TRY_RDLOCK(0, rwlock);
 139 #define ovs_rwlock_tryrdlock(rwlock) \
 140         ovs_rwlock_tryrdlock_at(rwlock, SOURCE_LOCATOR)
 141
 142 /* Wrappers for xpthread_cond_*() that abort the process on any error.
 143  *
 144  * Use ovs_mutex_cond_wait() to wait for a condition. */
 145 void xpthread_cond_init(pthread_cond_t *, pthread_condattr_t *);
 146 void xpthread_cond_destroy(pthread_cond_t *);
 147 void xpthread_cond_signal(pthread_cond_t *);
 148 void xpthread_cond_broadcast(pthread_cond_t *);
 149
 150 /* Wrappers for pthread_barrier_*() that abort the process on any error. */
 151 void xpthread_barrier_init(pthread_barrier_t *, pthread_barrierattr_t *,
 152                            unsigned int count);
 153 int xpthread_barrier_wait(pthread_barrier_t *);
 154 void xpthread_barrier_destroy(pthread_barrier_t *);
 155
 156 void xpthread_key_create(pthread_key_t *, void (*destructor)(void *));
 157 void xpthread_key_delete(pthread_key_t);
 158 void xpthread_setspecific(pthread_key_t, const void *);
 159
 160 pthread_t ovs_thread_create(const char *name, void *(*)(void *), void *);
 161 void xpthread_join(pthread_t, void **);
 162 \f
 163 /* Per-thread data.
 164  *
 165  *
 166  * Standard Forms
 167  * ==============
 168  *
 169  * Multiple forms of standard per-thread data exist, each with its own pluses
 170  * and minuses.  In general, if one of these forms is appropriate, then it's a
 171  * good idea to use it:
 172  *
 173  *     - POSIX per-thread data via pthread_key_t is portable to any pthreads
 174  *       implementation, and allows a destructor function to be defined.  It
 175  *       only (directly) supports per-thread pointers, which are always
 176  *       initialized to NULL.  It requires once-only allocation of a
 177  *       pthread_key_t value.  It is relatively slow.  Typically few
 178  *       "pthread_key_t"s are available (POSIX requires only at least 128,
 179  *       glibc supplies only 1024).
 180  *
 181  *     - The thread_local feature newly defined in C11 <threads.h> works with
 182  *       any data type and initializer, and it is fast.  thread_local does not
 183  *       require once-only initialization like pthread_key_t.  C11 does not
 184  *       define what happens if one attempts to access a thread_local object
 185  *       from a thread other than the one to which that object belongs.  There
 186  *       is no provision to call a user-specified destructor when a thread
 187  *       ends.  Typical implementations allow for an arbitrary amount of
 188  *       thread_local storage, but statically allocated only.
 189  *
 190  *     - The __thread keyword is a GCC extension similar to thread_local but
 191  *       with a longer history.  __thread is not portable to every GCC version
 192  *       or environment.  __thread does not restrict the use of a thread-local
 193  *       object outside its own thread.
 194  *
 195  * Here's a handy summary:
 196  *
 197  *                     pthread_key_t     thread_local       __thread
 198  *                     -------------     ------------     -------------
 199  * portability             high               low             medium
 200  * speed                    low              high               high
 201  * supports destructors?    yes                no                 no
 202  * needs key allocation?    yes                no                 no
 203  * arbitrary initializer?    no               yes                yes
 204  * cross-thread access?     yes                no                yes
 205  * amount available?        few            arbitrary         arbitrary
 206  * dynamically allocated?   yes                no                 no
 207  *
 208  *
 209  * Extensions
 210  * ==========
 211  *
 212  * OVS provides some extensions and wrappers:
 213  *
 214  *     - In a situation where the performance of thread_local or __thread is
 215  *       desirable, but portability is required, DEFINE_STATIC_PER_THREAD_DATA
 216  *       and DECLARE_EXTERN_PER_THREAD_DATA/DEFINE_EXTERN_PER_THREAD_DATA may
 217  *       be appropriate (see below).
 218  *
 219  *     - DEFINE_PER_THREAD_MALLOCED_DATA can be convenient for simple
 220  *       per-thread malloc()'d buffers.
 221  *
 222  *     - struct ovs_tsd provides an alternative to pthread_key_t that isn't
 223  *       limited to a small number of keys.
 224  */
 225
 226 /* For static data, use this macro in a source file:
 227  *
 228  *    DEFINE_STATIC_PER_THREAD_DATA(TYPE, NAME, INITIALIZER).
 229  *
 230  * For global data, "declare" the data in the header and "define" it in
 231  * the source file, with:
 232  *
 233  *    DECLARE_EXTERN_PER_THREAD_DATA(TYPE, NAME).
 234  *    DEFINE_EXTERN_PER_THREAD_DATA(NAME, INITIALIZER).
 235  *
 236  * One should prefer to use POSIX per-thread data, via pthread_key_t, when its
 237  * performance is acceptable, because of its portability (see the table above).
 238  * This macro is an alternatives that takes advantage of thread_local (and
 239  * __thread), for its performance, when it is available, and falls back to
 240  * POSIX per-thread data otherwise.
 241  *
 242  * Defines per-thread variable NAME with the given TYPE, initialized to
 243  * INITIALIZER (which must be valid as an initializer for a variable with
 244  * static lifetime).
 245  *
 246  * The public interface to the variable is:
 247  *
 248  *    TYPE *NAME_get(void)
 249  *    TYPE *NAME_get_unsafe(void)
 250  *
 251  *       Returns the address of this thread's instance of NAME.
 252  *
 253  *       Use NAME_get() in a context where this might be the first use of the
 254  *       per-thread variable in the program.  Use NAME_get_unsafe(), which
 255  *       avoids a conditional test and is thus slightly faster, in a context
 256  *       where one knows that NAME_get() has already been called previously.
 257  *
 258  * There is no "NAME_set()" (or "NAME_set_unsafe()") function.  To set the
 259  * value of the per-thread variable, dereference the pointer returned by
 260  * TYPE_get() or TYPE_get_unsafe(), e.g. *TYPE_get() = 0.
 261  */
 262 #if HAVE_THREAD_LOCAL || HAVE___THREAD
 263
 264 #if HAVE_THREAD_LOCAL
 265 #include <threads.h>
 266 #elif HAVE___THREAD
 267 #define thread_local __thread
 268 #else
 269 #error
 270 #endif
 271
 272 #define DEFINE_STATIC_PER_THREAD_DATA(TYPE, NAME, ...)                  \
 273     typedef TYPE NAME##_type;                                           \
 274                                                                         \
 275     static NAME##_type *                                                \
 276     NAME##_get_unsafe(void)                                             \
 277     {                                                                   \
 278         static thread_local NAME##_type var = __VA_ARGS__;              \
 279         return &var;                                                    \
 280     }                                                                   \
 281                                                                         \
 282     static NAME##_type *                                                \
 283     NAME##_get(void)                                                    \
 284     {                                                                   \
 285         return NAME##_get_unsafe();                                     \
 286     }
 287 #define DECLARE_EXTERN_PER_THREAD_DATA(TYPE, NAME)                      \
 288     typedef TYPE NAME##_type;                                           \
 289     extern thread_local NAME##_type NAME##_var;                         \
 290                                                                         \
 291     static inline NAME##_type *                                         \
 292     NAME##_get_unsafe(void)                                             \
 293     {                                                                   \
 294         return &NAME##_var;                                             \
 295     }                                                                   \
 296                                                                         \
 297     static inline NAME##_type *                                         \
 298     NAME##_get(void)                                                    \
 299     {                                                                   \
 300         return NAME##_get_unsafe();                                     \
 301     }
 302 #define DEFINE_EXTERN_PER_THREAD_DATA(NAME, ...)         \
 303     thread_local NAME##_type NAME##_var = __VA_ARGS__;
 304 #else  /* no C implementation support for thread-local storage  */
 305 #define DEFINE_STATIC_PER_THREAD_DATA(TYPE, NAME, ...)                  \
 306     typedef TYPE NAME##_type;                                           \
 307     static pthread_key_t NAME##_key;                                    \
 308                                                                         \
 309     static NAME##_type *                                                \
 310     NAME##_get_unsafe(void)                                             \
 311     {                                                                   \
 312         return pthread_getspecific(NAME##_key);                         \
 313     }                                                                   \
 314                                                                         \
 315     static void                                                         \
 316     NAME##_once_init(void)                                              \
 317     {                                                                   \
 318         if (pthread_key_create(&NAME##_key, free)) {                    \
 319             abort();                                                    \
 320         }                                                               \
 321     }                                                                   \
 322                                                                         \
 323     static NAME##_type *                                                \
 324     NAME##_get(void)                                                    \
 325     {                                                                   \
 326         static pthread_once_t once = PTHREAD_ONCE_INIT;                 \
 327         NAME##_type *value;                                             \
 328                                                                         \
 329         pthread_once(&once, NAME##_once_init);                          \
 330         value = NAME##_get_unsafe();                                    \
 331         if (!value) {                                                   \
 332             static const NAME##_type initial_value = __VA_ARGS__;       \
 333                                                                         \
 334             value = malloc(sizeof *value);                              \
 335             if (value == NULL) {                                        \
 336                 out_of_memory();                                        \
 337             }                                                           \
 338             *value = initial_value;                                     \
 339             xpthread_setspecific(NAME##_key, value);                    \
 340         }                                                               \
 341         return value;                                                   \
 342     }
 343 #define DECLARE_EXTERN_PER_THREAD_DATA(TYPE, NAME)                      \
 344     typedef TYPE NAME##_type;                                           \
 345     static pthread_key_t NAME##_key;                                    \
 346                                                                         \
 347     static inline NAME##_type *                                         \
 348     NAME##_get_unsafe(void)                                             \
 349     {                                                                   \
 350         return pthread_getspecific(NAME##_key);                         \
 351     }                                                                   \
 352                                                                         \
 353     NAME##_type *NAME##_get(void);
 354 #define DEFINE_EXTERN_PER_THREAD_DATA(NAME, ...)                        \
 355     static void                                                         \
 356     NAME##_once_init(void)                                              \
 357     {                                                                   \
 358         if (pthread_key_create(&NAME##_key, free)) {                    \
 359             abort();                                                    \
 360         }                                                               \
 361     }                                                                   \
 362                                                                         \
 363     NAME##_type *                                                       \
 364     NAME##_get(void)                                                    \
 365     {                                                                   \
 366         static pthread_once_t once = PTHREAD_ONCE_INIT;                 \
 367         NAME##_type *value;                                             \
 368                                                                         \
 369         pthread_once(&once, NAME##_once_init);                          \
 370         value = NAME##_get_unsafe();                                    \
 371         if (!value) {                                                   \
 372             static const NAME##_type initial_value = __VA_ARGS__;       \
 373                                                                         \
 374             value = malloc(sizeof *value);                              \
 375             if (value == NULL) {                                        \
 376                 out_of_memory();                                        \
 377             }                                                           \
 378             *value = initial_value;                                     \
 379             xpthread_setspecific(NAME##_key, value);                    \
 380         }                                                               \
 381         return value;                                                   \
 382     }
 383 #endif
 384
 385 /* DEFINE_PER_THREAD_MALLOCED_DATA(TYPE, NAME).
 386  *
 387  * This is a simple wrapper around POSIX per-thread data primitives.  It
 388  * defines per-thread variable NAME with the given TYPE, which must be a
 389  * pointer type.  In each thread, the per-thread variable is initialized to
 390  * NULL.  When a thread terminates, the variable is freed with free().
 391  *
 392  * The public interface to the variable is:
 393  *
 394  *    TYPE NAME_get(void)
 395  *    TYPE NAME_get_unsafe(void)
 396  *
 397  *       Returns the value of per-thread variable NAME in this thread.
 398  *
 399  *       Use NAME_get() in a context where this might be the first use of the
 400  *       per-thread variable in the program.  Use NAME_get_unsafe(), which
 401  *       avoids a conditional test and is thus slightly faster, in a context
 402  *       where one knows that NAME_get() has already been called previously.
 403  *
 404  *    TYPE NAME_set(TYPE new_value)
 405  *    TYPE NAME_set_unsafe(TYPE new_value)
 406  *
 407  *       Sets the value of per-thread variable NAME to 'new_value' in this
 408  *       thread, and returns its previous value.
 409  *
 410  *       Use NAME_set() in a context where this might be the first use of the
 411  *       per-thread variable in the program.  Use NAME_set_unsafe(), which
 412  *       avoids a conditional test and is thus slightly faster, in a context
 413  *       where one knows that NAME_set() has already been called previously.
 414  */
 415 #define DEFINE_PER_THREAD_MALLOCED_DATA(TYPE, NAME)     \
 416     static pthread_key_t NAME##_key;                    \
 417                                                         \
 418     static void                                         \
 419     NAME##_once_init(void)                              \
 420     {                                                   \
 421         if (pthread_key_create(&NAME##_key, free)) {    \
 422             abort();                                    \
 423         }                                               \
 424     }                                                   \
 425                                                         \
 426     static void                                         \
 427     NAME##_init(void)                                   \
 428     {                                                   \
 429         static pthread_once_t once = PTHREAD_ONCE_INIT; \
 430         pthread_once(&once, NAME##_once_init);          \
 431     }                                                   \
 432                                                         \
 433     static TYPE                                         \
 434     NAME##_get_unsafe(void)                             \
 435     {                                                   \
 436         return pthread_getspecific(NAME##_key);         \
 437     }                                                   \
 438                                                         \
 439     static OVS_UNUSED TYPE                              \
 440     NAME##_get(void)                                    \
 441     {                                                   \
 442         NAME##_init();                                  \
 443         return NAME##_get_unsafe();                     \
 444     }                                                   \
 445                                                         \
 446     static TYPE                                         \
 447     NAME##_set_unsafe(TYPE value)                       \
 448     {                                                   \
 449         TYPE old_value = NAME##_get_unsafe();           \
 450         xpthread_setspecific(NAME##_key, value);        \
 451         return old_value;                               \
 452     }                                                   \
 453                                                         \
 454     static OVS_UNUSED TYPE                              \
 455     NAME##_set(TYPE value)                              \
 456     {                                                   \
 457         NAME##_init();                                  \
 458         return NAME##_set_unsafe(value);                \
 459     }
 460
 461 /* Dynamically allocated thread-specific data with lots of slots.
 462  *
 463  * pthread_key_t can provide as few as 128 pieces of thread-specific data (even
 464  * glibc is limited to 1,024).  Thus, one must be careful to allocate only a
 465  * few keys globally.  One cannot, for example, allocate a key for every
 466  * instance of a data structure if there might be an arbitrary number of those
 467  * data structures.
 468  *
 469  * This API is similar to the pthread one (simply search and replace pthread_
 470  * by ovsthread_) but it a much larger limit that can be raised if necessary
 471  * (by recompiling).  Thus, one may more freely use this form of
 472  * thread-specific data.
 473  *
 474  * ovsthread_key_t also differs from pthread_key_t in the following ways:
 475  *
 476  *    - Destructors must not access thread-specific data (via ovsthread_key).
 477  *
 478  *    - The pthread_key_t API allows concurrently exiting threads to start
 479  *      executing the destructor after pthread_key_delete() returns.  The
 480  *      ovsthread_key_t API guarantees that, when ovsthread_key_delete()
 481  *      returns, all destructors have returned and no new ones will start
 482  *      execution.
 483  */
 484 typedef struct ovsthread_key *ovsthread_key_t;
 485
 486 void ovsthread_key_create(ovsthread_key_t *, void (*destructor)(void *));
 487 void ovsthread_key_delete(ovsthread_key_t);
 488
 489 void ovsthread_setspecific(ovsthread_key_t, const void *);
 490 void *ovsthread_getspecific(ovsthread_key_t);
 491 \f
 492 /* Convenient once-only execution.
 493  *
 494  *
 495  * Problem
 496  * =======
 497  *
 498  * POSIX provides pthread_once_t and pthread_once() as primitives for running a
 499  * set of code only once per process execution.  They are used like this:
 500  *
 501  *     static void run_once(void) { ...initialization... }
 502  *     static pthread_once_t once = PTHREAD_ONCE_INIT;
 503  * ...
 504  *     pthread_once(&once, run_once);
 505  *
 506  * pthread_once() does not allow passing any parameters to the initialization
 507  * function, which is often inconvenient, because it means that the function
 508  * can only access data declared at file scope.
 509  *
 510  *
 511  * Solution
 512  * ========
 513  *
 514  * Use ovsthread_once, like this, instead:
 515  *
 516  *     static struct ovsthread_once once = OVSTHREAD_ONCE_INITIALIZER;
 517  *
 518  *     if (ovsthread_once_start(&once)) {
 519  *         ...initialization...
 520  *         ovsthread_once_done(&once);
 521  *     }
 522  */
 523
 524 struct ovsthread_once {
 525     atomic_bool done;
 526     struct ovs_mutex mutex;
 527 };
 528
 529 #define OVSTHREAD_ONCE_INITIALIZER              \
 530     {                                           \
 531         ATOMIC_VAR_INIT(false),                 \
 532         OVS_MUTEX_INITIALIZER,                  \
 533     }
 534
 535 static inline bool ovsthread_once_start(struct ovsthread_once *once)
 536     OVS_TRY_LOCK(true, once->mutex);
 537 void ovsthread_once_done(struct ovsthread_once *once)
 538     OVS_RELEASES(once->mutex);
 539
 540 bool ovsthread_once_start__(struct ovsthread_once *once)
 541     OVS_TRY_LOCK(false, once->mutex);
 542
 543 static inline bool
 544 ovsthread_once_is_done__(struct ovsthread_once *once)
 545 {
 546     bool done;
 547
 548     atomic_read_explicit(&once->done, &done, memory_order_relaxed);
 549     return done;
 550 }
 551
 552 /* Returns true if this is the first call to ovsthread_once_start() for
 553  * 'once'.  In this case, the caller should perform whatever initialization
 554  * actions it needs to do, then call ovsthread_once_done() for 'once'.
 555  *
 556  * Returns false if this is not the first call to ovsthread_once_start() for
 557  * 'once'.  In this case, the call will not return until after
 558  * ovsthread_once_done() has been called. */
 559 static inline bool
 560 ovsthread_once_start(struct ovsthread_once *once)
 561 {
 562     return OVS_UNLIKELY(!ovsthread_once_is_done__(once)
 563                         && !ovsthread_once_start__(once));
 564 }
 565 \f
 566 /* Thread ID.
 567  *
 568  * pthread_t isn't so nice for some purposes.  Its size and representation are
 569  * implementation dependent, which means that there is no way to hash it.
 570  * This thread ID avoids the problem.
 571  */
 572
 573 DECLARE_EXTERN_PER_THREAD_DATA(unsigned int, ovsthread_id);
 574
 575 /* Returns a per-thread identifier unique within the lifetime of the
 576  * process. */
 577 static inline unsigned int
 578 ovsthread_id_self(void)
 579 {
 580     return *ovsthread_id_get();
 581 }
 582 \f
 583 /* Simulated global counter.
 584  *
 585  * Incrementing such a counter is meant to be cheaper than incrementing a
 586  * global counter protected by a lock.  It is probably more expensive than
 587  * incrementing a truly thread-local variable, but such a variable has no
 588  * straightforward way to get the sum.
 589  *
 590  *
 591  * Thread-safety
 592  * =============
 593  *
 594  * Fully thread-safe. */
 595
 596 struct ovsthread_stats {
 597     struct ovs_mutex mutex;
 598     void *volatile buckets[16];
 599 };
 600
 601 void ovsthread_stats_init(struct ovsthread_stats *);
 602 void ovsthread_stats_destroy(struct ovsthread_stats *);
 603
 604 void *ovsthread_stats_bucket_get(struct ovsthread_stats *,
 605                                  void *(*new_bucket)(void));
 606
 607 #define OVSTHREAD_STATS_FOR_EACH_BUCKET(BUCKET, IDX, STATS)             \
 608     for ((IDX) = ovs_thread_stats_next_bucket(STATS, 0);                \
 609          ((IDX) < ARRAY_SIZE((STATS)->buckets)                          \
 610           ? ((BUCKET) = (STATS)->buckets[IDX], true)                    \
 611           : false);                                                     \
 612          (IDX) = ovs_thread_stats_next_bucket(STATS, (IDX) + 1))
 613 size_t ovs_thread_stats_next_bucket(const struct ovsthread_stats *, size_t);
 614 \f
 615 bool single_threaded(void);
 616
 617 void assert_single_threaded_at(const char *where);
 618 #define assert_single_threaded() assert_single_threaded_at(SOURCE_LOCATOR)
 619
 620 #ifndef _WIN32
 621 pid_t xfork_at(const char *where);
 622 #define xfork() xfork_at(SOURCE_LOCATOR)
 623 #endif
 624
 625 void forbid_forking(const char *reason);
 626 bool may_fork(void);
 627 \f
 628 /* Useful functions related to threading. */
 629
 630 int count_cpu_cores(void);
 631
 632 #endif /* ovs-thread.h */