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