1 // Copyright (c) 2007, Google Inc. 2 // 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 6 // met: 7 // 8 // * Redistributions of source code must retain the above copyright 9 // notice, this list of conditions and the following disclaimer. 10 // * Redistributions in binary form must reproduce the above 11 // copyright notice, this list of conditions and the following disclaimer 12 // in the documentation and/or other materials provided with the 13 // distribution. 14 // * Neither the name of Google Inc. nor the names of its 15 // contributors may be used to endorse or promote products derived from 16 // this software without specific prior written permission. 17 // 18 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 19 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 20 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 21 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 22 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 23 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 24 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 25 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 26 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 27 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 28 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 29 // 30 // --- 31 // Author: Craig Silverstein. 32 // 33 // A simple mutex wrapper, supporting locks and read-write locks. 34 // You should assume the locks are *not* re-entrant. 35 // 36 // To use: you should define the following macros in your configure.ac: 37 // ACX_PTHREAD 38 // AC_RWLOCK 39 // The latter is defined in ../autoconf. 40 // 41 // This class is meant to be internal-only and should be wrapped by an 42 // internal namespace. Before you use this module, please give the 43 // name of your internal namespace for this module. Or, if you want 44 // to expose it, you'll want to move it to the Google namespace. We 45 // cannot put this class in global namespace because there can be some 46 // problems when we have multiple versions of Mutex in each shared object. 47 // 48 // NOTE: by default, we have #ifdef'ed out the TryLock() method. 49 // This is for two reasons: 50 // 1) TryLock() under Windows is a bit annoying (it requires a 51 // #define to be defined very early). 52 // 2) TryLock() is broken for NO_THREADS mode, at least in NDEBUG 53 // mode. 54 // If you need TryLock(), and either these two caveats are not a 55 // problem for you, or you're willing to work around them, then 56 // feel free to #define GMUTEX_TRYLOCK, or to remove the #ifdefs 57 // in the code below. 58 // 59 // CYGWIN NOTE: Cygwin support for rwlock seems to be buggy: 60 // http://www.cygwin.com/ml/cygwin/2008-12/msg00017.html 61 // Because of that, we might as well use windows locks for 62 // cygwin. They seem to be more reliable than the cygwin pthreads layer. 63 // 64 // TRICKY IMPLEMENTATION NOTE: 65 // This class is designed to be safe to use during 66 // dynamic-initialization -- that is, by global constructors that are 67 // run before main() starts. The issue in this case is that 68 // dynamic-initialization happens in an unpredictable order, and it 69 // could be that someone else's dynamic initializer could call a 70 // function that tries to acquire this mutex -- but that all happens 71 // before this mutex's constructor has run. (This can happen even if 72 // the mutex and the function that uses the mutex are in the same .cc 73 // file.) Basically, because Mutex does non-trivial work in its 74 // constructor, it's not, in the naive implementation, safe to use 75 // before dynamic initialization has run on it. 76 // 77 // The solution used here is to pair the actual mutex primitive with a 78 // bool that is set to true when the mutex is dynamically initialized. 79 // (Before that it's false.) Then we modify all mutex routines to 80 // look at the bool, and not try to lock/unlock until the bool makes 81 // it to true (which happens after the Mutex constructor has run.) 82 // 83 // This works because before main() starts -- particularly, during 84 // dynamic initialization -- there are no threads, so a) it's ok that 85 // the mutex operations are a no-op, since we don't need locking then 86 // anyway; and b) we can be quite confident our bool won't change 87 // state between a call to Lock() and a call to Unlock() (that would 88 // require a global constructor in one translation unit to call Lock() 89 // and another global constructor in another translation unit to call 90 // Unlock() later, which is pretty perverse). 91 // 92 // That said, it's tricky, and can conceivably fail; it's safest to 93 // avoid trying to acquire a mutex in a global constructor, if you 94 // can. One way it can fail is that a really smart compiler might 95 // initialize the bool to true at static-initialization time (too 96 // early) rather than at dynamic-initialization time. To discourage 97 // that, we set is_safe_ to true in code (not the constructor 98 // colon-initializer) and set it to true via a function that always 99 // evaluates to true, but that the compiler can't know always 100 // evaluates to true. This should be good enough. 101 // 102 // A related issue is code that could try to access the mutex 103 // after it's been destroyed in the global destructors (because 104 // the Mutex global destructor runs before some other global 105 // destructor, that tries to acquire the mutex). The way we 106 // deal with this is by taking a constructor arg that global 107 // mutexes should pass in, that causes the destructor to do no 108 // work. We still depend on the compiler not doing anything 109 // weird to a Mutex's memory after it is destroyed, but for a 110 // static global variable, that's pretty safe. 111 112 #ifndef GOOGLE_MUTEX_H_ 113 #define GOOGLE_MUTEX_H_ 114 115 #include "config.h" // to figure out pthreads support 116 117 #if defined(NO_THREADS) 118 typedef int MutexType; // to keep a lock-count 119 #elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__) 120 # define WIN32_LEAN_AND_MEAN // We only need minimal includes 121 # ifdef GMUTEX_TRYLOCK 122 // We need Windows NT or later for TryEnterCriticalSection(). If you 123 // don't need that functionality, you can remove these _WIN32_WINNT 124 // lines, and change TryLock() to assert(0) or something. 125 # ifndef _WIN32_WINNT 126 # define _WIN32_WINNT 0x0400 127 # endif 128 # endif 129 # include <windows.h> 130 typedef CRITICAL_SECTION MutexType; 131 #elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK) 132 // Needed for pthread_rwlock_*. If it causes problems, you could take it 133 // out, but then you'd have to unset HAVE_RWLOCK (at least on linux -- it 134 // *does* cause problems for FreeBSD, or MacOSX, but isn't needed 135 // for locking there.) 136 # ifdef __linux__ 137 # define _XOPEN_SOURCE 500 // may be needed to get the rwlock calls 138 # endif 139 # include <pthread.h> 140 typedef pthread_rwlock_t MutexType; 141 #elif defined(HAVE_PTHREAD) 142 # include <pthread.h> 143 typedef pthread_mutex_t MutexType; 144 #else 145 # error Need to implement mutex.h for your architecture, or #define NO_THREADS 146 #endif 147 148 #include <assert.h> 149 #include <stdlib.h> // for abort() 150 151 #define MUTEX_NAMESPACE gflags_mutex_namespace 152 153 namespace MUTEX_NAMESPACE { 154 155 class Mutex { 156 public: 157 // This is used for the single-arg constructor 158 enum LinkerInitialized { LINKER_INITIALIZED }; 159 160 // Create a Mutex that is not held by anybody. This constructor is 161 // typically used for Mutexes allocated on the heap or the stack. 162 inline Mutex(); 163 // This constructor should be used for global, static Mutex objects. 164 // It inhibits work being done by the destructor, which makes it 165 // safer for code that tries to acqiure this mutex in their global 166 // destructor. 167 inline Mutex(LinkerInitialized); 168 169 // Destructor 170 inline ~Mutex(); 171 172 inline void Lock(); // Block if needed until free then acquire exclusively 173 inline void Unlock(); // Release a lock acquired via Lock() 174 #ifdef GMUTEX_TRYLOCK 175 inline bool TryLock(); // If free, Lock() and return true, else return false 176 #endif 177 // Note that on systems that don't support read-write locks, these may 178 // be implemented as synonyms to Lock() and Unlock(). So you can use 179 // these for efficiency, but don't use them anyplace where being able 180 // to do shared reads is necessary to avoid deadlock. 181 inline void ReaderLock(); // Block until free or shared then acquire a share 182 inline void ReaderUnlock(); // Release a read share of this Mutex 183 inline void WriterLock() { Lock(); } // Acquire an exclusive lock 184 inline void WriterUnlock() { Unlock(); } // Release a lock from WriterLock() 185 186 private: 187 MutexType mutex_; 188 // We want to make sure that the compiler sets is_safe_ to true only 189 // when we tell it to, and never makes assumptions is_safe_ is 190 // always true. volatile is the most reliable way to do that. 191 volatile bool is_safe_; 192 // This indicates which constructor was called. 193 bool destroy_; 194 195 inline void SetIsSafe() { is_safe_ = true; } 196 197 // Catch the error of writing Mutex when intending MutexLock. 198 Mutex(Mutex* /*ignored*/) {} 199 // Disallow "evil" constructors 200 Mutex(const Mutex&); 201 void operator=(const Mutex&); 202 }; 203 204 // Now the implementation of Mutex for various systems 205 #if defined(NO_THREADS) 206 207 // When we don't have threads, we can be either reading or writing, 208 // but not both. We can have lots of readers at once (in no-threads 209 // mode, that's most likely to happen in recursive function calls), 210 // but only one writer. We represent this by having mutex_ be -1 when 211 // writing and a number > 0 when reading (and 0 when no lock is held). 212 // 213 // In debug mode, we assert these invariants, while in non-debug mode 214 // we do nothing, for efficiency. That's why everything is in an 215 // assert. 216 217 Mutex::Mutex() : mutex_(0) { } 218 Mutex::Mutex(Mutex::LinkerInitialized) : mutex_(0) { } 219 Mutex::~Mutex() { assert(mutex_ == 0); } 220 void Mutex::Lock() { assert(--mutex_ == -1); } 221 void Mutex::Unlock() { assert(mutex_++ == -1); } 222 #ifdef GMUTEX_TRYLOCK 223 bool Mutex::TryLock() { if (mutex_) return false; Lock(); return true; } 224 #endif 225 void Mutex::ReaderLock() { assert(++mutex_ > 0); } 226 void Mutex::ReaderUnlock() { assert(mutex_-- > 0); } 227 228 #elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__) 229 230 Mutex::Mutex() : destroy_(true) { 231 InitializeCriticalSection(&mutex_); 232 SetIsSafe(); 233 } 234 Mutex::Mutex(LinkerInitialized) : destroy_(false) { 235 InitializeCriticalSection(&mutex_); 236 SetIsSafe(); 237 } 238 Mutex::~Mutex() { if (destroy_) DeleteCriticalSection(&mutex_); } 239 void Mutex::Lock() { if (is_safe_) EnterCriticalSection(&mutex_); } 240 void Mutex::Unlock() { if (is_safe_) LeaveCriticalSection(&mutex_); } 241 #ifdef GMUTEX_TRYLOCK 242 bool Mutex::TryLock() { return is_safe_ ? 243 TryEnterCriticalSection(&mutex_) != 0 : true; } 244 #endif 245 void Mutex::ReaderLock() { Lock(); } // we don't have read-write locks 246 void Mutex::ReaderUnlock() { Unlock(); } 247 248 #elif defined(HAVE_PTHREAD) && defined(HAVE_RWLOCK) 249 250 #define SAFE_PTHREAD(fncall) do { /* run fncall if is_safe_ is true */ \ 251 if (is_safe_ && fncall(&mutex_) != 0) abort(); \ 252 } while (0) 253 254 Mutex::Mutex() : destroy_(true) { 255 SetIsSafe(); 256 if (is_safe_ && pthread_rwlock_init(&mutex_, NULL) != 0) abort(); 257 } 258 Mutex::Mutex(Mutex::LinkerInitialized) : destroy_(false) { 259 SetIsSafe(); 260 if (is_safe_ && pthread_rwlock_init(&mutex_, NULL) != 0) abort(); 261 } 262 Mutex::~Mutex() { if (destroy_) SAFE_PTHREAD(pthread_rwlock_destroy); } 263 void Mutex::Lock() { SAFE_PTHREAD(pthread_rwlock_wrlock); } 264 void Mutex::Unlock() { SAFE_PTHREAD(pthread_rwlock_unlock); } 265 #ifdef GMUTEX_TRYLOCK 266 bool Mutex::TryLock() { return is_safe_ ? 267 pthread_rwlock_trywrlock(&mutex_) == 0 : true; } 268 #endif 269 void Mutex::ReaderLock() { SAFE_PTHREAD(pthread_rwlock_rdlock); } 270 void Mutex::ReaderUnlock() { SAFE_PTHREAD(pthread_rwlock_unlock); } 271 #undef SAFE_PTHREAD 272 273 #elif defined(HAVE_PTHREAD) 274 275 #define SAFE_PTHREAD(fncall) do { /* run fncall if is_safe_ is true */ \ 276 if (is_safe_ && fncall(&mutex_) != 0) abort(); \ 277 } while (0) 278 279 Mutex::Mutex() : destroy_(true) { 280 SetIsSafe(); 281 if (is_safe_ && pthread_mutex_init(&mutex_, NULL) != 0) abort(); 282 } 283 Mutex::Mutex(Mutex::LinkerInitialized) : destroy_(false) { 284 SetIsSafe(); 285 if (is_safe_ && pthread_mutex_init(&mutex_, NULL) != 0) abort(); 286 } 287 Mutex::~Mutex() { if (destroy_) SAFE_PTHREAD(pthread_mutex_destroy); } 288 void Mutex::Lock() { SAFE_PTHREAD(pthread_mutex_lock); } 289 void Mutex::Unlock() { SAFE_PTHREAD(pthread_mutex_unlock); } 290 #ifdef GMUTEX_TRYLOCK 291 bool Mutex::TryLock() { return is_safe_ ? 292 pthread_mutex_trylock(&mutex_) == 0 : true; } 293 #endif 294 void Mutex::ReaderLock() { Lock(); } 295 void Mutex::ReaderUnlock() { Unlock(); } 296 #undef SAFE_PTHREAD 297 298 #endif 299 300 // -------------------------------------------------------------------------- 301 // Some helper classes 302 303 // MutexLock(mu) acquires mu when constructed and releases it when destroyed. 304 class MutexLock { 305 public: 306 explicit MutexLock(Mutex *mu) : mu_(mu) { mu_->Lock(); } 307 ~MutexLock() { mu_->Unlock(); } 308 private: 309 Mutex * const mu_; 310 // Disallow "evil" constructors 311 MutexLock(const MutexLock&); 312 void operator=(const MutexLock&); 313 }; 314 315 // ReaderMutexLock and WriterMutexLock do the same, for rwlocks 316 class ReaderMutexLock { 317 public: 318 explicit ReaderMutexLock(Mutex *mu) : mu_(mu) { mu_->ReaderLock(); } 319 ~ReaderMutexLock() { mu_->ReaderUnlock(); } 320 private: 321 Mutex * const mu_; 322 // Disallow "evil" constructors 323 ReaderMutexLock(const ReaderMutexLock&); 324 void operator=(const ReaderMutexLock&); 325 }; 326 327 class WriterMutexLock { 328 public: 329 explicit WriterMutexLock(Mutex *mu) : mu_(mu) { mu_->WriterLock(); } 330 ~WriterMutexLock() { mu_->WriterUnlock(); } 331 private: 332 Mutex * const mu_; 333 // Disallow "evil" constructors 334 WriterMutexLock(const WriterMutexLock&); 335 void operator=(const WriterMutexLock&); 336 }; 337 338 // Catch bug where variable name is omitted, e.g. MutexLock (&mu); 339 #define MutexLock(x) COMPILE_ASSERT(0, mutex_lock_decl_missing_var_name) 340 #define ReaderMutexLock(x) COMPILE_ASSERT(0, rmutex_lock_decl_missing_var_name) 341 #define WriterMutexLock(x) COMPILE_ASSERT(0, wmutex_lock_decl_missing_var_name) 342 343 } // namespace MUTEX_NAMESPACE 344 345 using namespace MUTEX_NAMESPACE; 346 347 #undef MUTEX_NAMESPACE 348 349 #endif /* #define GOOGLE_MUTEX_H__ */ 350