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

   1 // Copyright (C) 2002-2016 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
 111 // Test the guard variable with a memory load with
 112 // acquire semantics.
 113
 114 inline bool
 115 __test_and_acquire (__cxxabiv1::__guard *g)
 116 {
 117   unsigned char __c;
 118   unsigned char *__p = reinterpret_cast<unsigned char *>(g);
 119   __atomic_load (__p, &__c,  __ATOMIC_ACQUIRE);
 120   (void) __p;
 121   return _GLIBCXX_GUARD_TEST(&__c);
 122 }
 123 #  define _GLIBCXX_GUARD_TEST_AND_ACQUIRE(G) __test_and_acquire (G)
 124 # endif
 125
 126 # ifndef _GLIBCXX_GUARD_SET_AND_RELEASE
 127
 128 // Set the guard variable to 1 with memory order release semantics.
 129
 130 inline void
 131 __set_and_release (__cxxabiv1::__guard *g)
 132 {
 133   unsigned char *__p = reinterpret_cast<unsigned char *>(g);
 134   unsigned char val = 1;
 135   __atomic_store (__p, &val, __ATOMIC_RELEASE);
 136   (void) __p;
 137 }
 138 #  define _GLIBCXX_GUARD_SET_AND_RELEASE(G) __set_and_release (G)
 139 # endif
 140
 141 #else /* !__GTHREADS */
 142
 143 # undef _GLIBCXX_GUARD_TEST_AND_ACQUIRE
 144 # undef _GLIBCXX_GUARD_SET_AND_RELEASE
 145 # define _GLIBCXX_GUARD_SET_AND_RELEASE(G) _GLIBCXX_GUARD_SET (G)
 146
 147 #endif /* __GTHREADS */
 148
 149 //
 150 // Here are C++ run-time routines for guarded initialization of static
 151 // variables. There are 4 scenarios under which these routines are called:
 152 //
 153 //   1. Threads not supported (__GTHREADS not defined)
 154 //   2. Threads are supported but not enabled at run-time.
 155 //   3. Threads enabled at run-time but __gthreads_* are not fully POSIX.
 156 //   4. Threads enabled at run-time and __gthreads_* support all POSIX threads
 157 //      primitives we need here.
 158 //
 159 // The old code supported scenarios 1-3 but was broken since it used a global
 160 // mutex for all threads and had the mutex locked during the whole duration of
 161 // initialization of a guarded static variable. The following created a
 162 // dead-lock with the old code.
 163 //
 164 //      Thread 1 acquires the global mutex.
 165 //      Thread 1 starts initializing static variable.
 166 //      Thread 1 creates thread 2 during initialization.
 167 //      Thread 2 attempts to acquire mutex to initialize another variable.
 168 //      Thread 2 blocks since thread 1 is locking the mutex.
 169 //      Thread 1 waits for result from thread 2 and also blocks. A deadlock.
 170 //
 171 // The new code here can handle this situation and thus is more robust. However,
 172 // we need to use the POSIX thread condition variable, which is not supported
 173 // in all platforms, notably older versions of Microsoft Windows. The gthr*.h
 174 // headers define a symbol __GTHREAD_HAS_COND for platforms that support POSIX
 175 // like condition variables. For platforms that do not support condition
 176 // variables, we need to fall back to the old code.
 177
 178 // If _GLIBCXX_USE_FUTEX, no global mutex or condition variable is used,
 179 // only atomic operations are used together with futex syscall.
 180 // Valid values of the first integer in guard are:
 181 // 0                              No thread encountered the guarded init
 182 //                                yet or it has been aborted.
 183 // _GLIBCXX_GUARD_BIT             The guarded static var has been successfully
 184 //                                initialized.
 185 // _GLIBCXX_GUARD_PENDING_BIT     The guarded static var is being initialized
 186 //                                and no other thread is waiting for its
 187 //                                initialization.
 188 // (_GLIBCXX_GUARD_PENDING_BIT    The guarded static var is being initialized
 189 //  | _GLIBCXX_GUARD_WAITING_BIT) and some other threads are waiting until
 190 //                                it is initialized.
 191
 192 namespace __cxxabiv1
 193 {
 194 #ifdef _GLIBCXX_USE_FUTEX
 195   namespace
 196   {
 197     static inline int __guard_test_bit (const int __byte, const int __val)
 198     {
 199       union { int __i; char __c[sizeof (int)]; } __u = { 0 };
 200       __u.__c[__byte] = __val;
 201       return __u.__i;
 202     }
 203   }
 204 #endif
 205
 206   static inline int
 207   init_in_progress_flag(__guard* g)
 208   { return ((char *)g)[1]; }
 209
 210   static inline void
 211   set_init_in_progress_flag(__guard* g, int v)
 212   { ((char *)g)[1] = v; }
 213
 214   static inline void
 215   throw_recursive_init_exception()
 216   {
 217 #if __cpp_exceptions
 218         throw __gnu_cxx::recursive_init_error();
 219 #else
 220         // Use __builtin_trap so we don't require abort().
 221         __builtin_trap();
 222 #endif
 223   }
 224
 225   // acquire() is a helper function used to acquire guard if thread support is
 226   // not compiled in or is compiled in but not enabled at run-time.
 227   static int
 228   acquire(__guard *g)
 229   {
 230     // Quit if the object is already initialized.
 231     if (_GLIBCXX_GUARD_TEST(g))
 232       return 0;
 233
 234     if (init_in_progress_flag(g))
 235       throw_recursive_init_exception();
 236
 237     set_init_in_progress_flag(g, 1);
 238     return 1;
 239   }
 240
 241   extern "C"
 242   int __cxa_guard_acquire (__guard *g)
 243   {
 244 #ifdef __GTHREADS
 245     // If the target can reorder loads, we need to insert a read memory
 246     // barrier so that accesses to the guarded variable happen after the
 247     // guard test.
 248     if (_GLIBCXX_GUARD_TEST_AND_ACQUIRE (g))
 249       return 0;
 250
 251 # ifdef _GLIBCXX_USE_FUTEX
 252     // If __atomic_* and futex syscall are supported, don't use any global
 253     // mutex.
 254     if (__gthread_active_p ())
 255       {
 256         int *gi = (int *) (void *) g;
 257         const int guard_bit = _GLIBCXX_GUARD_BIT;
 258         const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
 259         const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 260
 261         while (1)
 262           {
 263             int expected(0);
 264             if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
 265                                             __ATOMIC_ACQ_REL,
 266                                             __ATOMIC_ACQUIRE))
 267               {
 268                 // This thread should do the initialization.
 269                 return 1;
 270               }
 271
 272             if (expected == guard_bit)
 273               {
 274                 // Already initialized.
 275                 return 0;
 276               }
 277
 278              if (expected == pending_bit)
 279                {
 280                  // Use acquire here.
 281                  int newv = expected | waiting_bit;
 282                  if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
 283                                                   __ATOMIC_ACQ_REL,
 284                                                   __ATOMIC_ACQUIRE))
 285                    {
 286                      if (expected == guard_bit)
 287                        {
 288                          // Make a thread that failed to set the
 289                          // waiting bit exit the function earlier,
 290                          // if it detects that another thread has
 291                          // successfully finished initialising.
 292                          return 0;
 293                        }
 294                      if (expected == 0)
 295                        continue;
 296                    }
 297
 298                  expected = newv;
 299                }
 300
 301             syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAIT, expected, 0);
 302           }
 303       }
 304 # else
 305     if (__gthread_active_p ())
 306       {
 307         mutex_wrapper mw;
 308
 309         while (1)       // When this loop is executing, mutex is locked.
 310           {
 311 #  ifdef __GTHREAD_HAS_COND
 312             // The static is already initialized.
 313             if (_GLIBCXX_GUARD_TEST(g))
 314               return 0; // The mutex will be unlocked via wrapper
 315
 316             if (init_in_progress_flag(g))
 317               {
 318                 // The guarded static is currently being initialized by
 319                 // another thread, so we release mutex and wait for the
 320                 // condition variable. We will lock the mutex again after
 321                 // this.
 322                 get_static_cond().wait_recursive(&get_static_mutex());
 323               }
 324             else
 325               {
 326                 set_init_in_progress_flag(g, 1);
 327                 return 1; // The mutex will be unlocked via wrapper.
 328               }
 329 #  else
 330             // This provides compatibility with older systems not supporting
 331             // POSIX like condition variables.
 332             if (acquire(g))
 333               {
 334                 mw.unlock = false;
 335                 return 1; // The mutex still locked.
 336               }
 337             return 0; // The mutex will be unlocked via wrapper.
 338 #  endif
 339           }
 340       }
 341 # endif
 342 #endif
 343
 344     return acquire (g);
 345   }
 346
 347   extern "C"
 348   void __cxa_guard_abort (__guard *g) throw ()
 349   {
 350 #ifdef _GLIBCXX_USE_FUTEX
 351     // If __atomic_* and futex syscall are supported, don't use any global
 352     // mutex.
 353     if (__gthread_active_p ())
 354       {
 355         int *gi = (int *) (void *) g;
 356         const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 357         int old = __atomic_exchange_n (gi, 0, __ATOMIC_ACQ_REL);
 358
 359         if ((old & waiting_bit) != 0)
 360           syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX);
 361         return;
 362       }
 363 #elif defined(__GTHREAD_HAS_COND)
 364     if (__gthread_active_p())
 365       {
 366         mutex_wrapper mw;
 367
 368         set_init_in_progress_flag(g, 0);
 369
 370         // If we abort, we still need to wake up all other threads waiting for
 371         // the condition variable.
 372         get_static_cond().broadcast();
 373         return;
 374       }
 375 #endif
 376
 377     set_init_in_progress_flag(g, 0);
 378 #if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND)
 379     // This provides compatibility with older systems not supporting POSIX like
 380     // condition variables.
 381     if (__gthread_active_p ())
 382       static_mutex->unlock();
 383 #endif
 384   }
 385
 386   extern "C"
 387   void __cxa_guard_release (__guard *g) throw ()
 388   {
 389 #ifdef _GLIBCXX_USE_FUTEX
 390     // If __atomic_* and futex syscall are supported, don't use any global
 391     // mutex.
 392     if (__gthread_active_p ())
 393       {
 394         int *gi = (int *) (void *) g;
 395         const int guard_bit = _GLIBCXX_GUARD_BIT;
 396         const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 397         int old = __atomic_exchange_n (gi, guard_bit, __ATOMIC_ACQ_REL);
 398
 399         if ((old & waiting_bit) != 0)
 400           syscall (SYS_futex, gi, _GLIBCXX_FUTEX_WAKE, INT_MAX);
 401         return;
 402       }
 403 #elif defined(__GTHREAD_HAS_COND)
 404     if (__gthread_active_p())
 405       {
 406         mutex_wrapper mw;
 407
 408         set_init_in_progress_flag(g, 0);
 409         _GLIBCXX_GUARD_SET_AND_RELEASE(g);
 410
 411         get_static_cond().broadcast();
 412         return;
 413       }
 414 #endif
 415
 416     set_init_in_progress_flag(g, 0);
 417     _GLIBCXX_GUARD_SET_AND_RELEASE (g);
 418
 419 #if defined(__GTHREADS) && !defined(__GTHREAD_HAS_COND)
 420     // This provides compatibility with older systems not supporting POSIX like
 421     // condition variables.
 422     if (__gthread_active_p())
 423       static_mutex->unlock();
 424 #endif
 425   }
 426 }