libstdc++-v3/libsupc++/guard.cc

   1 // Copyright (C) 2002, 2004, 2006, 2008, 2009, 2010, 2011, 2012
   2 // Free Software Foundation, Inc.
   3 //
   4 // This file is part of GCC.
   5 //
   6 // GCC is free software; you can redistribute it and/or modify
   7 // it under the terms of the GNU General Public License as published by
   8 // the Free Software Foundation; either version 3, or (at your option)
   9 // any later version.
  10
  11 // GCC is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15
  16 // Under Section 7 of GPL version 3, you are granted additional
  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
  20 // You should have received a copy of the GNU General Public License and
  21 // a copy of the GCC Runtime Library Exception along with this program;
  22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 // <http://www.gnu.org/licenses/>.
  24
  25 // Written by Mark Mitchell, CodeSourcery LLC, <mark@codesourcery.com>
  26 // Thread support written by Jason Merrill, Red Hat Inc. <jason@redhat.com>
  27
  28 #include <bits/c++config.h>
  29 #include <cxxabi.h>
  30 #include <exception>
  31 #include <new>
  32 #include <ext/atomicity.h>
  33 #include <ext/concurrence.h>
  34 #if defined(__GTHREADS) && defined(__GTHREAD_HAS_COND) \
  35   && (ATOMIC_INT_LOCK_FREE > 1) && defined(_GLIBCXX_HAVE_LINUX_FUTEX)
  36 # include <climits>
  37 # include <syscall.h>
  38 # include <unistd.h>
  39 # define _GLIBCXX_USE_FUTEX
  40 # define _GLIBCXX_FUTEX_WAIT 0
  41 # define _GLIBCXX_FUTEX_WAKE 1
  42 #endif
  43
  44 // The IA64/generic ABI uses the first byte of the guard variable.
  45 // The ARM EABI uses the least significant bit.
  46
  47 // Thread-safe static local initialization support.
  48 #ifdef __GTHREADS
  49 # ifndef _GLIBCXX_USE_FUTEX
  50 namespace
  51 {
  52   // A single mutex controlling all static initializations.
  53   static __gnu_cxx::__recursive_mutex* static_mutex;
  54
  55   typedef char fake_recursive_mutex[sizeof(__gnu_cxx::__recursive_mutex)]
  56   __attribute__ ((aligned(__alignof__(__gnu_cxx::__recursive_mutex))));
  57   fake_recursive_mutex fake_mutex;
  58
  59   static void init()
  60   { static_mutex =  new (&fake_mutex) __gnu_cxx::__recursive_mutex(); }
  61
  62   __gnu_cxx::__recursive_mutex&
  63   get_static_mutex()
  64   {
  65     static __gthread_once_t once = __GTHREAD_ONCE_INIT;
  66     __gthread_once(&once, init);
  67     return *static_mutex;
  68   }
  69
  70   // Simple wrapper for exception safety.
  71   struct mutex_wrapper
  72   {
  73     bool unlock;
  74     mutex_wrapper() : unlock(true)
  75     { get_static_mutex().lock(); }
  76
  77     ~mutex_wrapper()
  78     {
  79       if (unlock)
  80         static_mutex->unlock();
  81     }
  82   };
  83 }
  84 # endif
  85
  86 # if defined(__GTHREAD_HAS_COND) && !defined(_GLIBCXX_USE_FUTEX)
  87 namespace
  88 {
  89   // A single condition variable controlling all static initializations.
  90   static __gnu_cxx::__cond* static_cond;
  91
  92   // using a fake type to avoid initializing a static class.
  93   typedef char fake_cond_t[sizeof(__gnu_cxx::__cond)]
  94   __attribute__ ((aligned(__alignof__(__gnu_cxx::__cond))));
  95   fake_cond_t fake_cond;
  96
  97   static void init_static_cond()
  98   { static_cond =  new (&fake_cond) __gnu_cxx::__cond(); }
  99
 100   __gnu_cxx::__cond&
 101   get_static_cond()
 102   {
 103     static __gthread_once_t once = __GTHREAD_ONCE_INIT;
 104     __gthread_once(&once, init_static_cond);
 105     return *static_cond;
 106   }
 107 }
 108 # endif
 109
 110 # ifndef _GLIBCXX_GUARD_TEST_AND_ACQUIRE
 111 inline bool
 112 __test_and_acquire (__cxxabiv1::__guard *g)
 113 {
 114   bool b = _GLIBCXX_GUARD_TEST (g);
 115   _GLIBCXX_READ_MEM_BARRIER;
 116   return b;
 117 }
 118 #  define _GLIBCXX_GUARD_TEST_AND_ACQUIRE(G) __test_and_acquire (G)
 119 # endif
 120
 121 # ifndef _GLIBCXX_GUARD_SET_AND_RELEASE
 122 inline void
 123 __set_and_release (__cxxabiv1::__guard *g)
 124 {
 125   _GLIBCXX_WRITE_MEM_BARRIER;
 126   _GLIBCXX_GUARD_SET (g);
 127 }
 128 #  define _GLIBCXX_GUARD_SET_AND_RELEASE(G) __set_and_release (G)
 129 # endif
 130
 131 #else /* !__GTHREADS */
 132
 133 # undef _GLIBCXX_GUARD_TEST_AND_ACQUIRE
 134 # undef _GLIBCXX_GUARD_SET_AND_RELEASE
 135 # define _GLIBCXX_GUARD_SET_AND_RELEASE(G) _GLIBCXX_GUARD_SET (G)
 136
 137 #endif /* __GTHREADS */
 138
 139 //
 140 // Here are C++ run-time routines for guarded initialization of static
 141 // variables. There are 4 scenarios under which these routines are called:
 142 //
 143 //   1. Threads not supported (__GTHREADS not defined)
 144 //   2. Threads are supported but not enabled at run-time.
 145 //   3. Threads enabled at run-time but __gthreads_* are not fully POSIX.
 146 //   4. Threads enabled at run-time and __gthreads_* support all POSIX threads
 147 //      primitives we need here.
 148 //
 149 // The old code supported scenarios 1-3 but was broken since it used a global
 150 // mutex for all threads and had the mutex locked during the whole duration of
 151 // initialization of a guarded static variable. The following created a
 152 // dead-lock with the old code.
 153 //
 154 //      Thread 1 acquires the global mutex.
 155 //      Thread 1 starts initializing static variable.
 156 //      Thread 1 creates thread 2 during initialization.
 157 //      Thread 2 attempts to acquire mutex to initialize another variable.
 158 //      Thread 2 blocks since thread 1 is locking the mutex.
 159 //      Thread 1 waits for result from thread 2 and also blocks. A deadlock.
 160 //
 161 // The new code here can handle this situation and thus is more robust. However,
 162 // we need to use the POSIX thread condition variable, which is not supported
 163 // in all platforms, notably older versions of Microsoft Windows. The gthr*.h
 164 // headers define a symbol __GTHREAD_HAS_COND for platforms that support POSIX
 165 // like condition variables. For platforms that do not support condition
 166 // variables, we need to fall back to the old code.
 167
 168 // If _GLIBCXX_USE_FUTEX, no global mutex or condition variable is used,
 169 // only atomic operations are used together with futex syscall.
 170 // Valid values of the first integer in guard are:
 171 // 0                              No thread encountered the guarded init
 172 //                                yet or it has been aborted.
 173 // _GLIBCXX_GUARD_BIT             The guarded static var has been successfully
 174 //                                initialized.
 175 // _GLIBCXX_GUARD_PENDING_BIT     The guarded static var is being initialized
 176 //                                and no other thread is waiting for its
 177 //                                initialization.
 178 // (_GLIBCXX_GUARD_PENDING_BIT    The guarded static var is being initialized
 179 //  | _GLIBCXX_GUARD_WAITING_BIT) and some other threads are waiting until
 180 //                                it is initialized.
 181
 182 namespace __cxxabiv1
 183 {
 184 #ifdef _GLIBCXX_USE_FUTEX
 185   namespace
 186   {
 187     static inline int __guard_test_bit (const int __byte, const int __val)
 188     {
 189       union { int __i; char __c[sizeof (int)]; } __u = { 0 };
 190       __u.__c[__byte] = __val;
 191       return __u.__i;
 192     }
 193   }
 194 #endif
 195
 196   static inline int
 197   init_in_progress_flag(__guard* g)
 198   { return ((char *)g)[1]; }
 199
 200   static inline void
 201   set_init_in_progress_flag(__guard* g, int v)
 202   { ((char *)g)[1] = v; }
 203
 204   static inline void
 205   throw_recursive_init_exception()
 206   {
 207 #ifdef __EXCEPTIONS
 208         throw __gnu_cxx::recursive_init_error();
 209 #else
 210         // Use __builtin_trap so we don't require abort().
 211         __builtin_trap();
 212 #endif
 213   }
 214
 215   // acquire() is a helper function used to acquire guard if thread support is
 216   // not compiled in or is compiled in but not enabled at run-time.
 217   static int
 218   acquire(__guard *g)
 219   {
 220     // Quit if the object is already initialized.
 221     if (_GLIBCXX_GUARD_TEST(g))
 222       return 0;
 223
 224     if (init_in_progress_flag(g))
 225       throw_recursive_init_exception();
 226
 227     set_init_in_progress_flag(g, 1);
 228     return 1;
 229   }
 230
 231   extern "C"
 232   int __cxa_guard_acquire (__guard *g)
 233   {
 234 #ifdef __GTHREADS
 235     // If the target can reorder loads, we need to insert a read memory
 236     // barrier so that accesses to the guarded variable happen after the
 237     // guard test.
 238     if (_GLIBCXX_GUARD_TEST_AND_ACQUIRE (g))
 239       return 0;
 240
 241 # ifdef _GLIBCXX_USE_FUTEX
 242     // If __atomic_* and futex syscall are supported, don't use any global
 243     // mutex.
 244     if (__gthread_active_p ())
 245       {
 246         int *gi = (int *) (void *) g;
 247         int expected(0);
 248         const int guard_bit = _GLIBCXX_GUARD_BIT;
 249         const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
 250         const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 251
 252         while (1)
 253           {
 254             if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
 255                                             __ATOMIC_ACQ_REL,
 256                                             __ATOMIC_RELAXED))
 257               {
 258                 // This thread should do the initialization.
 259                 return 1;
 260               }
 261
 262             if (expected == guard_bit)
 263               {
 264                 // Already initialized.
 265                 return 0;
 266               }
 267              if (expected == pending_bit)
 268                {
 269                  int newv = expected | waiting_bit;
 270                  if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
 271                                                   __ATOMIC_ACQ_REL,
 272                                                   __ATOMIC_RELAXED))
 273                    continue;
 274
 275                  expected = newv;
 276                }
 277
 278             syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAIT, expected, 0);
 279           }
 280       }
 281 # else
 282     if (__gthread_active_p ())
 283       {
 284         mutex_wrapper mw;
 285
 286         while (1)       // When this loop is executing, mutex is locked.
 287           {
 288 #  ifdef __GTHREAD_HAS_COND
 289             // The static is already initialized.
 290             if (_GLIBCXX_GUARD_TEST(g))
 291               return 0; // The mutex will be unlocked via wrapper
 292
 293             if (init_in_progress_flag(g))
 294               {
 295                 // The guarded static is currently being initialized by
 296                 // another thread, so we release mutex and wait for the
 297                 // condition variable. We will lock the mutex again after
 298                 // this.
 299                 get_static_cond().wait_recursive(&get_static_mutex());
 300               }
 301             else
 302               {
 303                 set_init_in_progress_flag(g, 1);
 304                 return 1; // The mutex will be unlocked via wrapper.
 305               }
 306 #  else
 307             // This provides compatibility with older systems not supporting
 308             // POSIX like condition variables.
 309             if (acquire(g))
 310               {
 311                 mw.unlock = false;
 312                 return 1; // The mutex still locked.
 313               }
 314             return 0; // The mutex will be unlocked via wrapper.
 315 #  endif
 316           }
 317       }
 318 # endif
 319 #endif
 320
 321     return acquire (g);
 322   }
 323
 324   extern "C"
 325   void __cxa_guard_abort (__guard *g) throw ()
 326   {
 327 #ifdef _GLIBCXX_USE_FUTEX
 328     // If __atomic_* and futex syscall are supported, don't use any global
 329     // mutex.
 330     if (__gthread_active_p ())
 331       {
 332         int *gi = (int *) (void *) g;
 333         const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 334         int old = __atomic_exchange_n (gi, 0, __ATOMIC_ACQ_REL);
 335
 336         if ((old & waiting_bit) != 0)
 337           syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX);
 338         return;
 339       }
 340 #elif defined(__GTHREAD_HAS_COND)
 341     if (__gthread_active_p())
 342       {
 343         mutex_wrapper mw;
 344
 345         set_init_in_progress_flag(g, 0);
 346
 347         // If we abort, we still need to wake up all other threads waiting for
 348         // the condition variable.
 349         get_static_cond().broadcast();
 350         return;
 351       }
 352 #endif
 353
 354     set_init_in_progress_flag(g, 0);
 355 #if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND)
 356     // This provides compatibility with older systems not supporting POSIX like
 357     // condition variables.
 358     if (__gthread_active_p ())
 359       static_mutex->unlock();
 360 #endif
 361   }
 362
 363   extern "C"
 364   void __cxa_guard_release (__guard *g) throw ()
 365   {
 366 #ifdef _GLIBCXX_USE_FUTEX
 367     // If __atomic_* and futex syscall are supported, don't use any global
 368     // mutex.
 369     if (__gthread_active_p ())
 370       {
 371         int *gi = (int *) (void *) g;
 372         const int guard_bit = _GLIBCXX_GUARD_BIT;
 373         const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 374         int old = __atomic_exchange_n (gi, guard_bit, __ATOMIC_ACQ_REL);
 375
 376         if ((old & waiting_bit) != 0)
 377           syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX);
 378         return;
 379       }
 380 #elif defined(__GTHREAD_HAS_COND)
 381     if (__gthread_active_p())
 382       {
 383         mutex_wrapper mw;
 384
 385         set_init_in_progress_flag(g, 0);
 386         _GLIBCXX_GUARD_SET_AND_RELEASE(g);
 387
 388         get_static_cond().broadcast();
 389         return;
 390       }
 391 #endif
 392
 393     set_init_in_progress_flag(g, 0);
 394     _GLIBCXX_GUARD_SET_AND_RELEASE (g);
 395
 396 #if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND)
 397     // This provides compatibility with older systems not supporting POSIX like
 398     // condition variables.
 399     if (__gthread_active_p())
 400       static_mutex->unlock();
 401 #endif
 402   }
 403 }