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

   1 // Copyright (C) 2002-2014 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
  49 namespace
  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)
  86 namespace
  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
 110 inline 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
 121 inline 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
 181 namespace __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 #ifdef __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 }