libobjc/THREADS

   1 This file describes in little detail the modifications to the
   2 Objective-C runtime needed to make it thread safe.
   3
   4 First off, kudos to Galen Hunt who is the author of this great work.
   5
   6 If you have an comments or just want to know where to
   7 send me money to express your undying gratitude for threading the
   8 Objective-C runtime you can reach Galen at:
   9
  10         gchunt@cs.rochester.edu
  11
  12 Any questions, comments, bug reports, etc. should send email either to the
  13 GCC bug account or to:
  14
  15         Scott Christley <scottc@net-community.com>
  16
  17 * Sarray Threading:
  18
  19 The most critical component of the Objective-C runtime is the sparse array
  20 structure (sarray).  Sarrays store object selectors and implementations.
  21 Following in the tradition of the Objective-C runtime, my threading
  22 support assumes that fast message dispatching is far more important
  23 than *ANY* and *ALL* other operations.  The message dispatching thus
  24 uses *NO* locks on any kind.  In fact, if you look in sarray.h, you
  25 will notice that the message dispatching has not been modified.
  26 Instead, I have modified the sarray management functions so that all
  27 updates to the sarray data structure can be made in parallel will
  28 message dispatching.
  29
  30 To support concurrent message dispatching, no dynamically allocated
  31 sarray data structures are freed while more than one thread is
  32 operational.  Sarray data structures that are no longer in use are
  33 kept in a linked list of garbage and are released whenever the program
  34 is operating with a single thread.  The programmer can also flush the
  35 garbage list by calling sarray_remove_garbage when the programmer can
  36 ensure that no message dispatching is taking place concurrently.  The
  37 amount of un-reclaimed sarray garbage should normally be extremely
  38 small in a real program as sarray structures are freed only when using
  39 the "poseAs" functionality and early in program initialization, which
  40 normally occurs while the program is single threaded.
  41
  42 ******************************************************************************
  43 * Static Variables:
  44
  45 The following variables are either statically or globally defined. This list
  46 does not include variables which are internal to implementation dependent
  47 versions of thread-*.c.
  48
  49 The following threading designations are used:
  50         SAFE   : Implicitly thread safe.
  51         SINGLE : Must only be used in single thread mode.
  52         MUTEX  : Protected by single global mutex objc_runtime_mutex.
  53         UNUSED : Not used in the runtime.
  54
  55 Variable Name:                  Usage:  Defined:        Also used in:
  56 ===========================     ======  ============    =====================
  57 __objc_class_hash               MUTEX   class.c
  58 __objc_class_links_resolved     UNUSED  class.c         runtime.h
  59 __objc_class_number             MUTEX   class.c
  60 __objc_dangling_categories      UNUSED  init.c
  61 __objc_module_list              MUTEX   init.c
  62 __objc_selector_array           MUTEX   selector.c
  63 __objc_selector_hash            MUTEX   selector.c
  64 __objc_selector_max_index       MUTEX   selector.c      sendmsg.c runtime.h
  65 __objc_selector_names           MUTEX   selector.c
  66 __objc_thread_exit_status       SAFE    thread.c
  67 __objc_uninstalled_dtable       MUTEX   sendmsg.c       selector.c
  68 _objc_load_callback             SAFE    init.c          objc-api.h
  69 _objc_lookup_class              SAFE    class.c         objc-api.h
  70 _objc_object_alloc              SINGLE  objects.c       objc-api.h
  71 _objc_object_copy               SINGLE  objects.c       objc-api.h
  72 _objc_object_dispose            SINGLE  objects.c       objc-api.h
  73 frwd_sel                        SAFE2   sendmsg.c
  74 idxsize                         MUTEX   sarray.c        sendmsg.c sarray.h
  75 initialize_sel                  SAFE2   sendmsg.c
  76 narrays                         MUTEX   sarray.c        sendmsg.c sarray.h
  77 nbuckets                        MUTEX   sarray.c        sendmsg.c sarray.h
  78 nindices                        MUTEX   sarray.c        sarray.h
  79 previous_constructors           SAFE1   init.c
  80 proto_class                     SAFE1   init.c
  81 unclaimed_categories            MUTEX   init.c
  82 unclaimed_proto_list            MUTEX   init.c
  83 uninitialized_statics           MUTEX   init.c
  84
  85 Notes:
  86 1) Initialized once in unithread mode.
  87 2) Initialized value will always be same, guaranteed by lock on selector
  88    hash table.
  89
  90
  91 ******************************************************************************
  92 * Frontend/Backend design:
  93
  94 The design of the Objective-C runtime thread and mutex functions utilizes a
  95 frontend/backend implementation.
  96
  97 The frontend, as characterized by the files thr.h and thr.c, is a set
  98 of platform independent structures and functions which represent the
  99 user interface.  Objective-C programs should use these structures and
 100 functions for their thread and mutex work if they wish to maintain a
 101 high degree of portability across platforms.
 102
 103 The backend is composed of a file with the necessary code to map the ObjC
 104 thread and mutex to a platform specific implementation.  For example, the
 105 file thr-solaris.c contains the implementation for Solaris.
 106
 107 If you are compiling libobjc as part of GCC, the thr-objc.c backend is
 108 always used; this backend uses GCC's gthread code.  The thread system
 109 is automatically configured when GCC is configured.  Important: make
 110 sure you configure GCC using `--enable-threads' if you want threads !
 111
 112 If you want to compile libobjc standalone, then you would need to
 113 modify the configure.in and makefiles for it; and you need to pick an
 114 appropriate backend file for the target platform; you make this choice
 115 by assigning the OBJC_THREAD_FILE make variable to the basename of the
 116 backend file.  For example, OBJC_THREAD_FILE=thr-posix would indicate
 117 that the generic posix backend file, thr-posix.c, should be compiled
 118 with the ObjC runtime library.  If your platform does not support
 119 threads then you should specify the OBJC_THREAD_FILE=thr-single
 120 backend file to compile the ObjC runtime library without thread or
 121 mutex support; note that programs which rely upon the ObjC thread and
 122 mutex functions will compile and link correctly but attempting to
 123 create a thread or mutex will result in an error.
 124
 125 It is questionable whether it is really necessary to have both a
 126 frontend and backend function for all available functionality.  On the
 127 one hand, it provides a clear, consistent differentiation between what
 128 is public and what is private with the downside of having the overhead
 129 of multiple functions calls.  For example, the function to have a
 130 thread yield the processor is objc_thread_yield; in the current
 131 implementation this produces a function call set:
 132
 133 objc_thread_yield()  ->  __objc_thread_yield()  ->  system yield function
 134
 135 This has two extra function calls over calling the platform specific function
 136 explicitly, but the issue is whether only the overhead of a single function
 137 is necessary.
 138
 139 objc_thread_yield()  ->  system yield function
 140
 141 This breaks the public/private dichotomy between the frontend/backend
 142 for the sake of efficiency.  It is possible to just use a preprocessor
 143 define so as to eliminate the extra function call:
 144
 145 #define objc_thread_yield() __objc_thread_yield()
 146
 147 This has the undesirable effect that if objc_thread_yield is actually
 148 turned into a function based upon future need; then ObjC programs which
 149 access the thread functions would need to be recompiled versus just
 150 being relinked.
 151
 152 ******************************************************************************
 153 * Threads:
 154
 155 The thread system attempts to create multiple threads using whatever
 156 operating system or library thread support is available.  It does
 157 assume that all system functions are thread safe.  Notably this means
 158 that the system implementation of malloc and free must be thread safe.
 159 If a system has multiple processors, the threads are configured for
 160 full parallel processing.
 161
 162 * Backend initialization functions
 163
 164 __objc_init_thread_system(void), int
 165         Initialize the thread subsystem.  Called once by __objc_exec_class.
 166         Return -1 if error otherwise return 0.
 167
 168 __objc_close_thread_system(void), int
 169         Closes the thread subsystem, not currently guaranteed to be called.
 170         Return -1 if error otherwise return 0.
 171
 172 *****
 173 * Frontend thread functions
 174 * User programs should use these functions.
 175
 176 objc_thread_detach(SEL selector, id object, id argument), objc_thread_t
 177         Creates and detaches a new thread.  The new thread starts by
 178         sending the given selector with a single argument to the
 179         given object.
 180
 181 objc_thread_set_priority(int priority), int
 182         Sets a thread's relative priority within the program.  Valid
 183         options are:
 184
 185         OBJC_THREAD_INTERACTIVE_PRIORITY
 186         OBJC_THREAD_BACKGROUND_PRIORITY
 187         OBJC_THREAD_LOW_PRIORITY
 188
 189 objc_thread_get_priority(void), int
 190         Query a thread's priority.
 191
 192 objc_thread_yield(void), void
 193         Yields processor to another thread with equal or higher
 194         priority.  It is up to the system scheduler to determine if
 195         the processor is taken or not.
 196
 197 objc_thread_exit(void), int
 198         Terminates a thread.  If this is the last thread executing
 199         then the program will terminate.
 200
 201 objc_thread_id(void), int
 202         Returns the current thread's id.
 203
 204 objc_thread_set_data(void *value), int
 205         Set a pointer to the thread's local storage.  Local storage is
 206         thread specific.
 207
 208 objc_thread_get_data(void), void *
 209         Returns the pointer to the thread's local storage.
 210
 211 *****
 212 * Backend thread functions
 213 * User programs should *NOT* directly call these functions.
 214
 215 __objc_thread_detach(void (*func)(void *arg), void *arg), objc_thread_t
 216         Spawns a new thread executing func, called by objc_thread_detach.
 217         Return NULL if error otherwise return thread id.
 218
 219 __objc_thread_set_priority(int priority), int
 220         Set the thread's priority, called by objc_thread_set_priority.
 221         Return -1 if error otherwise return 0.
 222
 223 __objc_thread_get_priority(void), int
 224         Query a thread's priority, called by objc_thread_get_priority.
 225         Return -1 if error otherwise return the priority.
 226
 227 __objc_thread_yield(void), void
 228         Yields the processor, called by objc_thread_yield.
 229
 230 __objc_thread_exit(void), int
 231         Terminates the thread, called by objc_thread_exit.
 232         Return -1 if error otherwise function does not return.
 233
 234 __objc_thread_id(void), objc_thread_t
 235         Returns the current thread's id, called by objc_thread_id.
 236         Return -1 if error otherwise return thread id.
 237
 238 __objc_thread_set_data(void *value), int
 239         Set pointer for thread local storage, called by objc_thread_set_data.
 240         Returns -1 if error otherwise return 0.
 241
 242 __objc_thread_get_data(void), void *
 243         Returns the pointer to the thread's local storage.
 244         Returns NULL if error, called by objc_thread_get_data.
 245
 246
 247 ******************************************************************************
 248 * Mutexes:
 249
 250 Mutexes can be locked recursively.  Each locked mutex remembers
 251 its owner (by thread id) and how many times it has been locked.  The
 252 last unlock on a mutex removes the system lock and allows other
 253 threads to access the mutex.
 254
 255 *****
 256 * Frontend mutex functions
 257 * User programs should use these functions.
 258
 259 objc_mutex_allocate(void), objc_mutex_t
 260         Allocates a new mutex.  Mutex is initially unlocked.
 261         Return NULL if error otherwise return mutex pointer.
 262
 263 objc_mutex_deallocate(objc_mutex_t mutex), int
 264         Free a mutex.  Before freeing the mutex, makes sure that no
 265         one else is using it.
 266         Return -1 if error otherwise return 0.
 267
 268 objc_mutex_lock(objc_mutex_t mutex), int
 269         Locks a mutex.  As mentioned earlier, the same thread may call
 270         this routine repeatedly.
 271         Return -1 if error otherwise return 0.
 272
 273 objc_mutex_trylock(objc_mutex_t mutex), int
 274         Attempts to lock a mutex.  If lock on mutex can be acquired
 275         then function operates exactly as objc_mutex_lock.
 276         Return -1 if failed to acquire lock otherwise return 0.
 277
 278 objc_mutex_unlock(objc_mutex_t mutex), int
 279         Unlocks the mutex by one level.  Other threads may not acquire
 280         the mutex until this thread has released all locks on it.
 281         Return -1 if error otherwise return 0.
 282
 283 *****
 284 * Backend mutex functions
 285 * User programs should *NOT* directly call these functions.
 286
 287 __objc_mutex_allocate(objc_mutex_t mutex), int
 288         Allocates a new mutex, called by objc_mutex_allocate.
 289         Return -1 if error otherwise return 0.
 290
 291 __objc_mutex_deallocate(objc_mutex_t mutex), int
 292         Free a mutex, called by objc_mutex_deallocate.
 293         Return -1 if error otherwise return 0.
 294
 295 __objc_mutex_lock(objc_mutex_t mutex), int
 296         Locks a mutex, called by objc_mutex_lock.
 297         Return -1 if error otherwise return 0.
 298
 299 __objc_mutex_trylock(objc_mutex_t mutex), int
 300         Attempts to lock a mutex, called by objc_mutex_trylock.
 301         Return -1 if failed to acquire lock or error otherwise return 0.
 302
 303 __objc_mutex_unlock(objc_mutex_t mutex), int
 304         Unlocks the mutex, called by objc_mutex_unlock.
 305         Return -1 if error otherwise return 0.
 306
 307 ******************************************************************************
 308 * Condition Mutexes:
 309
 310 Mutexes can be locked recursively.  Each locked mutex remembers
 311 its owner (by thread id) and how many times it has been locked.  The
 312 last unlock on a mutex removes the system lock and allows other
 313 threads to access the mutex.
 314
 315 *
 316 * Frontend condition mutex functions
 317 * User programs should use these functions.
 318 *
 319
 320 objc_condition_allocate(void), objc_condition_t
 321         Allocate a condition mutex.
 322         Return NULL if error otherwise return condition pointer.
 323
 324 objc_condition_deallocate(objc_condition_t condition), int
 325         Deallocate a condition. Note that this includes an implicit
 326         condition_broadcast to insure that waiting threads have the
 327         opportunity to wake.  It is legal to dealloc a condition only
 328         if no other thread is/will be using it. Does NOT check for
 329         other threads waiting but just wakes them up.
 330         Return -1 if error otherwise return 0.
 331
 332 objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int
 333         Wait on the condition unlocking the mutex until objc_condition_signal()
 334         or objc_condition_broadcast() are called for the same condition. The
 335         given mutex *must* have the depth 1 so that it can be unlocked
 336         here, for someone else can lock it and signal/broadcast the condition.
 337         The mutex is used to lock access to the shared data that make up the
 338         "condition" predicate.
 339         Return -1 if error otherwise return 0.
 340
 341 objc_condition_broadcast(objc_condition_t condition), int
 342         Wake up all threads waiting on this condition. It is recommended that
 343         the called would lock the same mutex as the threads in
 344         objc_condition_wait before changing the "condition predicate"
 345         and make this call and unlock it right away after this call.
 346         Return -1 if error otherwise return 0.
 347
 348 objc_condition_signal(objc_condition_t condition), int
 349         Wake up one thread waiting on this condition.
 350         Return -1 if error otherwise return 0.
 351
 352 *
 353 * Backend condition mutex functions
 354 * User programs should *NOT* directly call these functions.
 355 *
 356
 357 __objc_condition_allocate(objc_condition_t condition), int
 358         Allocate a condition mutex, called by objc_condition_allocate.
 359         Return -1 if error otherwise return 0.
 360
 361 __objc_condition_deallocate(objc_condition_t condition), int
 362         Deallocate a condition, called by objc_condition_deallocate.
 363         Return -1 if error otherwise return 0.
 364
 365 __objc_condition_wait(objc_condition_t condition, objc_mutex_t mutex), int
 366         Wait on the condition, called by objc_condition_wait.
 367         Return -1 if error otherwise return 0 when condition is met.
 368
 369 __objc_condition_broadcast(objc_condition_t condition), int
 370         Wake up all threads waiting on this condition.
 371         Called by objc_condition_broadcast.
 372         Return -1 if error otherwise return 0.
 373
 374 __objc_condition_signal(objc_condition_t condition), int
 375         Wake up one thread waiting on this condition.
 376         Called by objc_condition_signal.
 377         Return -1 if error otherwise return 0.