guard.cc revision 1.5
1// Copyright (C) 2002-2015 Free Software Foundation, Inc. 2// 3// This file is part of GCC. 4// 5// GCC is free software; you can redistribute it and/or modify 6// it under the terms of the GNU General Public License as published by 7// the Free Software Foundation; either version 3, or (at your option) 8// any later version. 9 10// GCC is distributed in the hope that it will be useful, 11// but WITHOUT ANY WARRANTY; without even the implied warranty of 12// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 13// GNU General Public License for more details. 14 15// Under Section 7 of GPL version 3, you are granted additional 16// permissions described in the GCC Runtime Library Exception, version 17// 3.1, as published by the Free Software Foundation. 18 19// You should have received a copy of the GNU General Public License and 20// a copy of the GCC Runtime Library Exception along with this program; 21// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see 22// <http://www.gnu.org/licenses/>. 23 24// Written by Mark Mitchell, CodeSourcery LLC, <mark@codesourcery.com> 25// Thread support written by Jason Merrill, Red Hat Inc. <jason@redhat.com> 26 27#include <bits/c++config.h> 28#include <cxxabi.h> 29#include <exception> 30#include <new> 31#include <ext/atomicity.h> 32#include <ext/concurrence.h> 33#if defined(__GTHREADS) && defined(__GTHREAD_HAS_COND) \ 34 && (ATOMIC_INT_LOCK_FREE > 1) && defined(_GLIBCXX_HAVE_LINUX_FUTEX) 35# include <climits> 36# include <syscall.h> 37# include <unistd.h> 38# define _GLIBCXX_USE_FUTEX 39# define _GLIBCXX_FUTEX_WAIT 0 40# define _GLIBCXX_FUTEX_WAKE 1 41#endif 42 43// The IA64/generic ABI uses the first byte of the guard variable. 44// The ARM EABI uses the least significant bit. 45 46// Thread-safe static local initialization support. 47#ifdef __GTHREADS 48# ifndef _GLIBCXX_USE_FUTEX 49namespace 50{ 51 // A single mutex controlling all static initializations. 52 static __gnu_cxx::__recursive_mutex* static_mutex; 53 54 typedef char fake_recursive_mutex[sizeof(__gnu_cxx::__recursive_mutex)] 55 __attribute__ ((aligned(__alignof__(__gnu_cxx::__recursive_mutex)))); 56 fake_recursive_mutex fake_mutex; 57 58 static void init() 59 { static_mutex = new (&fake_mutex) __gnu_cxx::__recursive_mutex(); } 60 61 __gnu_cxx::__recursive_mutex& 62 get_static_mutex() 63 { 64 static __gthread_once_t once = __GTHREAD_ONCE_INIT; 65 __gthread_once(&once, init); 66 return *static_mutex; 67 } 68 69 // Simple wrapper for exception safety. 70 struct mutex_wrapper 71 { 72 bool unlock; 73 mutex_wrapper() : unlock(true) 74 { get_static_mutex().lock(); } 75 76 ~mutex_wrapper() 77 { 78 if (unlock) 79 static_mutex->unlock(); 80 } 81 }; 82} 83# endif 84 85# if defined(__GTHREAD_HAS_COND) && !defined(_GLIBCXX_USE_FUTEX) 86namespace 87{ 88 // A single condition variable controlling all static initializations. 89 static __gnu_cxx::__cond* static_cond; 90 91 // using a fake type to avoid initializing a static class. 92 typedef char fake_cond_t[sizeof(__gnu_cxx::__cond)] 93 __attribute__ ((aligned(__alignof__(__gnu_cxx::__cond)))); 94 fake_cond_t fake_cond; 95 96 static void init_static_cond() 97 { static_cond = new (&fake_cond) __gnu_cxx::__cond(); } 98 99 __gnu_cxx::__cond& 100 get_static_cond() 101 { 102 static __gthread_once_t once = __GTHREAD_ONCE_INIT; 103 __gthread_once(&once, init_static_cond); 104 return *static_cond; 105 } 106} 107# endif 108 109# ifndef _GLIBCXX_GUARD_TEST_AND_ACQUIRE 110inline bool 111__test_and_acquire (__cxxabiv1::__guard *g) 112{ 113 bool b = _GLIBCXX_GUARD_TEST (g); 114 _GLIBCXX_READ_MEM_BARRIER; 115 return b; 116} 117# define _GLIBCXX_GUARD_TEST_AND_ACQUIRE(G) __test_and_acquire (G) 118# endif 119 120# ifndef _GLIBCXX_GUARD_SET_AND_RELEASE 121inline void 122__set_and_release (__cxxabiv1::__guard *g) 123{ 124 _GLIBCXX_WRITE_MEM_BARRIER; 125 _GLIBCXX_GUARD_SET (g); 126} 127# define _GLIBCXX_GUARD_SET_AND_RELEASE(G) __set_and_release (G) 128# endif 129 130#else /* !__GTHREADS */ 131 132# undef _GLIBCXX_GUARD_TEST_AND_ACQUIRE 133# undef _GLIBCXX_GUARD_SET_AND_RELEASE 134# define _GLIBCXX_GUARD_SET_AND_RELEASE(G) _GLIBCXX_GUARD_SET (G) 135 136#endif /* __GTHREADS */ 137 138// 139// Here are C++ run-time routines for guarded initialization of static 140// variables. There are 4 scenarios under which these routines are called: 141// 142// 1. Threads not supported (__GTHREADS not defined) 143// 2. Threads are supported but not enabled at run-time. 144// 3. Threads enabled at run-time but __gthreads_* are not fully POSIX. 145// 4. Threads enabled at run-time and __gthreads_* support all POSIX threads 146// primitives we need here. 147// 148// The old code supported scenarios 1-3 but was broken since it used a global 149// mutex for all threads and had the mutex locked during the whole duration of 150// initialization of a guarded static variable. The following created a 151// dead-lock with the old code. 152// 153// Thread 1 acquires the global mutex. 154// Thread 1 starts initializing static variable. 155// Thread 1 creates thread 2 during initialization. 156// Thread 2 attempts to acquire mutex to initialize another variable. 157// Thread 2 blocks since thread 1 is locking the mutex. 158// Thread 1 waits for result from thread 2 and also blocks. A deadlock. 159// 160// The new code here can handle this situation and thus is more robust. However, 161// we need to use the POSIX thread condition variable, which is not supported 162// in all platforms, notably older versions of Microsoft Windows. The gthr*.h 163// headers define a symbol __GTHREAD_HAS_COND for platforms that support POSIX 164// like condition variables. For platforms that do not support condition 165// variables, we need to fall back to the old code. 166 167// If _GLIBCXX_USE_FUTEX, no global mutex or condition variable is used, 168// only atomic operations are used together with futex syscall. 169// Valid values of the first integer in guard are: 170// 0 No thread encountered the guarded init 171// yet or it has been aborted. 172// _GLIBCXX_GUARD_BIT The guarded static var has been successfully 173// initialized. 174// _GLIBCXX_GUARD_PENDING_BIT The guarded static var is being initialized 175// and no other thread is waiting for its 176// initialization. 177// (_GLIBCXX_GUARD_PENDING_BIT The guarded static var is being initialized 178// | _GLIBCXX_GUARD_WAITING_BIT) and some other threads are waiting until 179// it is initialized. 180 181namespace __cxxabiv1 182{ 183#ifdef _GLIBCXX_USE_FUTEX 184 namespace 185 { 186 static inline int __guard_test_bit (const int __byte, const int __val) 187 { 188 union { int __i; char __c[sizeof (int)]; } __u = { 0 }; 189 __u.__c[__byte] = __val; 190 return __u.__i; 191 } 192 } 193#endif 194 195 static inline int 196 init_in_progress_flag(__guard* g) 197 { return ((char *)g)[1]; } 198 199 static inline void 200 set_init_in_progress_flag(__guard* g, int v) 201 { ((char *)g)[1] = v; } 202 203 static inline void 204 throw_recursive_init_exception() 205 { 206#if __cpp_exceptions 207 throw __gnu_cxx::recursive_init_error(); 208#else 209 // Use __builtin_trap so we don't require abort(). 210 __builtin_trap(); 211#endif 212 } 213 214 // acquire() is a helper function used to acquire guard if thread support is 215 // not compiled in or is compiled in but not enabled at run-time. 216 static int 217 acquire(__guard *g) 218 { 219 // Quit if the object is already initialized. 220 if (_GLIBCXX_GUARD_TEST(g)) 221 return 0; 222 223 if (init_in_progress_flag(g)) 224 throw_recursive_init_exception(); 225 226 set_init_in_progress_flag(g, 1); 227 return 1; 228 } 229 230 extern "C" 231 int __cxa_guard_acquire (__guard *g) 232 { 233#ifdef __GTHREADS 234 // If the target can reorder loads, we need to insert a read memory 235 // barrier so that accesses to the guarded variable happen after the 236 // guard test. 237 if (_GLIBCXX_GUARD_TEST_AND_ACQUIRE (g)) 238 return 0; 239 240# ifdef _GLIBCXX_USE_FUTEX 241 // If __atomic_* and futex syscall are supported, don't use any global 242 // mutex. 243 if (__gthread_active_p ()) 244 { 245 int *gi = (int *) (void *) g; 246 const int guard_bit = _GLIBCXX_GUARD_BIT; 247 const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT; 248 const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT; 249 250 while (1) 251 { 252 int expected(0); 253 if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false, 254 __ATOMIC_ACQ_REL, 255 __ATOMIC_ACQUIRE)) 256 { 257 // This thread should do the initialization. 258 return 1; 259 } 260 261 if (expected == guard_bit) 262 { 263 // Already initialized. 264 return 0; 265 } 266 267 if (expected == pending_bit) 268 { 269 // Use acquire here. 270 int newv = expected | waiting_bit; 271 if (!__atomic_compare_exchange_n(gi, &expected, newv, false, 272 __ATOMIC_ACQ_REL, 273 __ATOMIC_ACQUIRE)) 274 { 275 if (expected == guard_bit) 276 { 277 // Make a thread that failed to set the 278 // waiting bit exit the function earlier, 279 // if it detects that another thread has 280 // successfully finished initialising. 281 return 0; 282 } 283 if (expected == 0) 284 continue; 285 } 286 287 expected = newv; 288 } 289 290 syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAIT, expected, 0); 291 } 292 } 293# else 294 if (__gthread_active_p ()) 295 { 296 mutex_wrapper mw; 297 298 while (1) // When this loop is executing, mutex is locked. 299 { 300# ifdef __GTHREAD_HAS_COND 301 // The static is already initialized. 302 if (_GLIBCXX_GUARD_TEST(g)) 303 return 0; // The mutex will be unlocked via wrapper 304 305 if (init_in_progress_flag(g)) 306 { 307 // The guarded static is currently being initialized by 308 // another thread, so we release mutex and wait for the 309 // condition variable. We will lock the mutex again after 310 // this. 311 get_static_cond().wait_recursive(&get_static_mutex()); 312 } 313 else 314 { 315 set_init_in_progress_flag(g, 1); 316 return 1; // The mutex will be unlocked via wrapper. 317 } 318# else 319 // This provides compatibility with older systems not supporting 320 // POSIX like condition variables. 321 if (acquire(g)) 322 { 323 mw.unlock = false; 324 return 1; // The mutex still locked. 325 } 326 return 0; // The mutex will be unlocked via wrapper. 327# endif 328 } 329 } 330# endif 331#endif 332 333 return acquire (g); 334 } 335 336 extern "C" 337 void __cxa_guard_abort (__guard *g) throw () 338 { 339#ifdef _GLIBCXX_USE_FUTEX 340 // If __atomic_* and futex syscall are supported, don't use any global 341 // mutex. 342 if (__gthread_active_p ()) 343 { 344 int *gi = (int *) (void *) g; 345 const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT; 346 int old = __atomic_exchange_n (gi, 0, __ATOMIC_ACQ_REL); 347 348 if ((old & waiting_bit) != 0) 349 syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX); 350 return; 351 } 352#elif defined(__GTHREAD_HAS_COND) 353 if (__gthread_active_p()) 354 { 355 mutex_wrapper mw; 356 357 set_init_in_progress_flag(g, 0); 358 359 // If we abort, we still need to wake up all other threads waiting for 360 // the condition variable. 361 get_static_cond().broadcast(); 362 return; 363 } 364#endif 365 366 set_init_in_progress_flag(g, 0); 367#if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND) 368 // This provides compatibility with older systems not supporting POSIX like 369 // condition variables. 370 if (__gthread_active_p ()) 371 static_mutex->unlock(); 372#endif 373 } 374 375 extern "C" 376 void __cxa_guard_release (__guard *g) throw () 377 { 378#ifdef _GLIBCXX_USE_FUTEX 379 // If __atomic_* and futex syscall are supported, don't use any global 380 // mutex. 381 if (__gthread_active_p ()) 382 { 383 int *gi = (int *) (void *) g; 384 const int guard_bit = _GLIBCXX_GUARD_BIT; 385 const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT; 386 int old = __atomic_exchange_n (gi, guard_bit, __ATOMIC_ACQ_REL); 387 388 if ((old & waiting_bit) != 0) 389 syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX); 390 return; 391 } 392#elif defined(__GTHREAD_HAS_COND) 393 if (__gthread_active_p()) 394 { 395 mutex_wrapper mw; 396 397 set_init_in_progress_flag(g, 0); 398 _GLIBCXX_GUARD_SET_AND_RELEASE(g); 399 400 get_static_cond().broadcast(); 401 return; 402 } 403#endif 404 405 set_init_in_progress_flag(g, 0); 406 _GLIBCXX_GUARD_SET_AND_RELEASE (g); 407 408#if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND) 409 // This provides compatibility with older systems not supporting POSIX like 410 // condition variables. 411 if (__gthread_active_p()) 412 static_mutex->unlock(); 413#endif 414 } 415} 416