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