src/libs/libcxxrt/guard.cc

   1 /*
   2  * Copyright 2010-2012 PathScale, Inc. All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are met:
   6  *
   7  * 1. Redistributions of source code must retain the above copyright notice,
   8  *    this list of conditions and the following disclaimer.
   9  *
  10  * 2. Redistributions in binary form must reproduce the above copyright notice,
  11  *    this list of conditions and the following disclaimer in the documentation
  12  *    and/or other materials provided with the distribution.
  13  *
  14  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS
  15  * IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
  16  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  17  * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
  18  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  19  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  20  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
  21  * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
  22  * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
  23  * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  24  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  25  */
  26
  27 /**
  28  * guard.cc: Functions for thread-safe static initialisation.
  29  *
  30  * Static values in C++ can be initialised lazily their first use.  This file
  31  * contains functions that are used to ensure that two threads attempting to
  32  * initialize the same static do not call the constructor twice.  This is
  33  * important because constructors can have side effects, so calling the
  34  * constructor twice may be very bad.
  35  *
  36  * Statics that require initialisation are protected by a 64-bit value.  Any
  37  * platform that can do 32-bit atomic test and set operations can use this
  38  * value as a low-overhead lock.  Because statics (in most sane code) are
  39  * accessed far more times than they are initialised, this lock implementation
  40  * is heavily optimised towards the case where the static has already been
  41  * initialised.
  42  */
  43 #include <stdint.h>
  44 #include <stdlib.h>
  45 #include <stdio.h>
  46 #include <pthread.h>
  47 #include <assert.h>
  48 #include "atomic.h"
  49
  50 // Older GCC doesn't define __LITTLE_ENDIAN__
  51 #ifndef __LITTLE_ENDIAN__
  52         // If __BYTE_ORDER__ is defined, use that instead
  53 #       ifdef __BYTE_ORDER__
  54 #               if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
  55 #                       define __LITTLE_ENDIAN__
  56 #               endif
  57         // x86 and ARM are the most common little-endian CPUs, so let's have a
  58         // special case for them (ARM is already special cased).  Assume everything
  59         // else is big endian.
  60 #       elif defined(__x86_64) || defined(__i386)
  61 #               define __LITTLE_ENDIAN__
  62 #       endif
  63 #endif
  64
  65
  66 /*
  67  * The least significant bit of the guard variable indicates that the object
  68  * has been initialised, the most significant bit is used for a spinlock.
  69  */
  70 #ifdef __arm__
  71 // ARM ABI - 32-bit guards.
  72 typedef uint32_t guard_t;
  73 typedef uint32_t guard_lock_t;
  74 static const uint32_t LOCKED = static_cast<guard_t>(1) << 31;
  75 static const uint32_t INITIALISED = 1;
  76 #define LOCK_PART(guard) (guard)
  77 #define INIT_PART(guard) (guard)
  78 #elif defined(_LP64)
  79 typedef uint64_t guard_t;
  80 typedef uint64_t guard_lock_t;
  81 #       if defined(__LITTLE_ENDIAN__)
  82 static const guard_t LOCKED = static_cast<guard_t>(1) << 63;
  83 static const guard_t INITIALISED = 1;
  84 #       else
  85 static const guard_t LOCKED = 1;
  86 static const guard_t INITIALISED = static_cast<guard_t>(1) << 56;
  87 #       endif
  88 #define LOCK_PART(guard) (guard)
  89 #define INIT_PART(guard) (guard)
  90 #else
  91 typedef uint32_t guard_lock_t;
  92 #       if defined(__LITTLE_ENDIAN__)
  93 typedef struct {
  94         uint32_t init_half;
  95         uint32_t lock_half;
  96 } guard_t;
  97 static const uint32_t LOCKED = static_cast<guard_lock_t>(1) << 31;
  98 static const uint32_t INITIALISED = 1;
  99 #       else
 100 typedef struct {
 101         uint32_t init_half;
 102         uint32_t lock_half;
 103 } guard_t;
 104 static_assert(sizeof(guard_t) == sizeof(uint64_t), "");
 105 static const uint32_t LOCKED = 1;
 106 static const uint32_t INITIALISED = static_cast<guard_lock_t>(1) << 24;
 107 #       endif
 108 #define LOCK_PART(guard) (&(guard)->lock_half)
 109 #define INIT_PART(guard) (&(guard)->init_half)
 110 #endif
 111 static const guard_lock_t INITIAL = 0;
 112
 113 /**
 114  * Acquires a lock on a guard, returning 0 if the object has already been
 115  * initialised, and 1 if it has not.  If the object is already constructed then
 116  * this function just needs to read a byte from memory and return.
 117  */
 118 extern "C" int __cxa_guard_acquire(volatile guard_t *guard_object)
 119 {
 120         guard_lock_t old;
 121         // Not an atomic read, doesn't establish a happens-before relationship, but
 122         // if one is already established and we end up seeing an initialised state
 123         // then it's a fast path, otherwise we'll do something more expensive than
 124         // this test anyway...
 125         if (INITIALISED == *INIT_PART(guard_object))
 126                 return 0;
 127         // Spin trying to do the initialisation
 128         for (;;)
 129         {
 130                 // Loop trying to move the value of the guard from 0 (not
 131                 // locked, not initialised) to the locked-uninitialised
 132                 // position.
 133                 old = __sync_val_compare_and_swap(LOCK_PART(guard_object),
 134                     INITIAL, LOCKED);
 135                 if (old == INITIAL) {
 136                         // Lock obtained.  If lock and init bit are
 137                         // in separate words, check for init race.
 138                         if (INIT_PART(guard_object) == LOCK_PART(guard_object))
 139                                 return 1;
 140                         if (INITIALISED != *INIT_PART(guard_object))
 141                                 return 1;
 142
 143                         // No need for a memory barrier here,
 144                         // see first comment.
 145                         *LOCK_PART(guard_object) = INITIAL;
 146                         return 0;
 147                 }
 148                 // If lock and init bit are in the same word, check again
 149                 // if we are done.
 150                 if (INIT_PART(guard_object) == LOCK_PART(guard_object) &&
 151                     old == INITIALISED)
 152                         return 0;
 153
 154                 assert(old == LOCKED);
 155                 // Another thread holds the lock.
 156                 // If lock and init bit are in different words, check
 157                 // if we are done before yielding and looping.
 158                 if (INIT_PART(guard_object) != LOCK_PART(guard_object) &&
 159                     INITIALISED == *INIT_PART(guard_object))
 160                         return 0;
 161                 sched_yield();
 162         }
 163 }
 164
 165 /**
 166  * Releases the lock without marking the object as initialised.  This function
 167  * is called if initialising a static causes an exception to be thrown.
 168  */
 169 extern "C" void __cxa_guard_abort(volatile guard_t *guard_object)
 170 {
 171         __attribute__((unused))
 172         bool reset = __sync_bool_compare_and_swap(LOCK_PART(guard_object),
 173             LOCKED, INITIAL);
 174         assert(reset);
 175 }
 176 /**
 177  * Releases the guard and marks the object as initialised.  This function is
 178  * called after successful initialisation of a static.
 179  */
 180 extern "C" void __cxa_guard_release(volatile guard_t *guard_object)
 181 {
 182         guard_lock_t old;
 183         if (INIT_PART(guard_object) == LOCK_PART(guard_object))
 184                 old = LOCKED;
 185         else
 186                 old = INITIAL;
 187         __attribute__((unused))
 188         bool reset = __sync_bool_compare_and_swap(INIT_PART(guard_object),
 189             old, INITIALISED);
 190         assert(reset);
 191         if (INIT_PART(guard_object) != LOCK_PART(guard_object))
 192                 *LOCK_PART(guard_object) = INITIAL;
 193 }