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