thread_local_storage.cc revision cce46a0c214b37e8da48c522c83037e8ffa4f9fd
1// Copyright 2014 The Chromium Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style license that can be 3// found in the LICENSE file. 4 5#include "base/threading/thread_local_storage.h" 6 7#include "base/atomicops.h" 8#include "base/logging.h" 9#include "build/build_config.h" 10 11using base::internal::PlatformThreadLocalStorage; 12 13namespace { 14// In order to make TLS destructors work, we need to keep around a function 15// pointer to the destructor for each slot. We keep this array of pointers in a 16// global (static) array. 17// We use the single OS-level TLS slot (giving us one pointer per thread) to 18// hold a pointer to a per-thread array (table) of slots that we allocate to 19// Chromium consumers. 20 21// g_native_tls_key is the one native TLS that we use. It stores our table. 22base::subtle::Atomic32 g_native_tls_key = 23 PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES; 24 25// g_last_used_tls_key is the high-water-mark of allocated thread local storage. 26// Each allocation is an index into our g_tls_destructors[]. Each such index is 27// assigned to the instance variable slot_ in a ThreadLocalStorage::Slot 28// instance. We reserve the value slot_ == 0 to indicate that the corresponding 29// instance of ThreadLocalStorage::Slot has been freed (i.e., destructor called, 30// etc.). This reserved use of 0 is then stated as the initial value of 31// g_last_used_tls_key, so that the first issued index will be 1. 32base::subtle::Atomic32 g_last_used_tls_key = 0; 33 34// The maximum number of 'slots' in our thread local storage stack. 35const int kThreadLocalStorageSize = 256; 36 37// The maximum number of times to try to clear slots by calling destructors. 38// Use pthread naming convention for clarity. 39const int kMaxDestructorIterations = kThreadLocalStorageSize; 40 41// An array of destructor function pointers for the slots. If a slot has a 42// destructor, it will be stored in its corresponding entry in this array. 43// The elements are volatile to ensure that when the compiler reads the value 44// to potentially call the destructor, it does so once, and that value is tested 45// for null-ness and then used. Yes, that would be a weird de-optimization, 46// but I can imagine some register machines where it was just as easy to 47// re-fetch an array element, and I want to be sure a call to free the key 48// (i.e., null out the destructor entry) that happens on a separate thread can't 49// hurt the racy calls to the destructors on another thread. 50volatile base::ThreadLocalStorage::TLSDestructorFunc 51 g_tls_destructors[kThreadLocalStorageSize]; 52 53// This function is called to initialize our entire Chromium TLS system. 54// It may be called very early, and we need to complete most all of the setup 55// (initialization) before calling *any* memory allocator functions, which may 56// recursively depend on this initialization. 57// As a result, we use Atomics, and avoid anything (like a singleton) that might 58// require memory allocations. 59void** ConstructTlsVector() { 60 PlatformThreadLocalStorage::TLSKey key = 61 base::subtle::NoBarrier_Load(&g_native_tls_key); 62 if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) { 63 CHECK(PlatformThreadLocalStorage::AllocTLS(&key)); 64 65 // The TLS_KEY_OUT_OF_INDEXES is used to find out whether the key is set or 66 // not in NoBarrier_CompareAndSwap, but Posix doesn't have invalid key, we 67 // define an almost impossible value be it. 68 // If we really get TLS_KEY_OUT_OF_INDEXES as value of key, just alloc 69 // another TLS slot. 70 if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) { 71 PlatformThreadLocalStorage::TLSKey tmp = key; 72 CHECK(PlatformThreadLocalStorage::AllocTLS(&key) && 73 key != PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES); 74 PlatformThreadLocalStorage::FreeTLS(tmp); 75 } 76 // Atomically test-and-set the tls_key. If the key is 77 // TLS_KEY_OUT_OF_INDEXES, go ahead and set it. Otherwise, do nothing, as 78 // another thread already did our dirty work. 79 if (PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES != 80 static_cast<PlatformThreadLocalStorage::TLSKey>( 81 base::subtle::NoBarrier_CompareAndSwap( 82 &g_native_tls_key, 83 PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES, key))) { 84 // We've been shortcut. Another thread replaced g_native_tls_key first so 85 // we need to destroy our index and use the one the other thread got 86 // first. 87 PlatformThreadLocalStorage::FreeTLS(key); 88 key = base::subtle::NoBarrier_Load(&g_native_tls_key); 89 } 90 } 91 CHECK(!PlatformThreadLocalStorage::GetTLSValue(key)); 92 93 // Some allocators, such as TCMalloc, make use of thread local storage. 94 // As a result, any attempt to call new (or malloc) will lazily cause such a 95 // system to initialize, which will include registering for a TLS key. If we 96 // are not careful here, then that request to create a key will call new back, 97 // and we'll have an infinite loop. We avoid that as follows: 98 // Use a stack allocated vector, so that we don't have dependence on our 99 // allocator until our service is in place. (i.e., don't even call new until 100 // after we're setup) 101 void* stack_allocated_tls_data[kThreadLocalStorageSize]; 102 memset(stack_allocated_tls_data, 0, sizeof(stack_allocated_tls_data)); 103 // Ensure that any rentrant calls change the temp version. 104 PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data); 105 106 // Allocate an array to store our data. 107 void** tls_data = new void*[kThreadLocalStorageSize]; 108 memcpy(tls_data, stack_allocated_tls_data, sizeof(stack_allocated_tls_data)); 109 PlatformThreadLocalStorage::SetTLSValue(key, tls_data); 110 return tls_data; 111} 112 113void OnThreadExitInternal(void* value) { 114 DCHECK(value); 115 void** tls_data = static_cast<void**>(value); 116 // Some allocators, such as TCMalloc, use TLS. As a result, when a thread 117 // terminates, one of the destructor calls we make may be to shut down an 118 // allocator. We have to be careful that after we've shutdown all of the 119 // known destructors (perchance including an allocator), that we don't call 120 // the allocator and cause it to resurrect itself (with no possibly destructor 121 // call to follow). We handle this problem as follows: 122 // Switch to using a stack allocated vector, so that we don't have dependence 123 // on our allocator after we have called all g_tls_destructors. (i.e., don't 124 // even call delete[] after we're done with destructors.) 125 void* stack_allocated_tls_data[kThreadLocalStorageSize]; 126 memcpy(stack_allocated_tls_data, tls_data, sizeof(stack_allocated_tls_data)); 127 // Ensure that any re-entrant calls change the temp version. 128 PlatformThreadLocalStorage::TLSKey key = 129 base::subtle::NoBarrier_Load(&g_native_tls_key); 130 PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data); 131 delete[] tls_data; // Our last dependence on an allocator. 132 133 int remaining_attempts = kMaxDestructorIterations; 134 bool need_to_scan_destructors = true; 135 while (need_to_scan_destructors) { 136 need_to_scan_destructors = false; 137 // Try to destroy the first-created-slot (which is slot 1) in our last 138 // destructor call. That user was able to function, and define a slot with 139 // no other services running, so perhaps it is a basic service (like an 140 // allocator) and should also be destroyed last. If we get the order wrong, 141 // then we'll itterate several more times, so it is really not that 142 // critical (but it might help). 143 base::subtle::Atomic32 last_used_tls_key = 144 base::subtle::NoBarrier_Load(&g_last_used_tls_key); 145 for (int slot = last_used_tls_key; slot > 0; --slot) { 146 void* tls_value = stack_allocated_tls_data[slot]; 147 if (tls_value == NULL) 148 continue; 149 150 base::ThreadLocalStorage::TLSDestructorFunc destructor = 151 g_tls_destructors[slot]; 152 if (destructor == NULL) 153 continue; 154 stack_allocated_tls_data[slot] = NULL; // pre-clear the slot. 155 destructor(tls_value); 156 // Any destructor might have called a different service, which then set 157 // a different slot to a non-NULL value. Hence we need to check 158 // the whole vector again. This is a pthread standard. 159 need_to_scan_destructors = true; 160 } 161 if (--remaining_attempts <= 0) { 162 NOTREACHED(); // Destructors might not have been called. 163 break; 164 } 165 } 166 167 // Remove our stack allocated vector. 168 PlatformThreadLocalStorage::SetTLSValue(key, NULL); 169} 170 171} // namespace 172 173namespace base { 174 175namespace internal { 176 177#if defined(OS_WIN) 178void PlatformThreadLocalStorage::OnThreadExit() { 179 PlatformThreadLocalStorage::TLSKey key = 180 base::subtle::NoBarrier_Load(&g_native_tls_key); 181 if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) 182 return; 183 void *tls_data = GetTLSValue(key); 184 // Maybe we have never initialized TLS for this thread. 185 if (!tls_data) 186 return; 187 OnThreadExitInternal(tls_data); 188} 189#elif defined(OS_POSIX) 190void PlatformThreadLocalStorage::OnThreadExit(void* value) { 191 OnThreadExitInternal(value); 192} 193#endif // defined(OS_WIN) 194 195} // namespace internal 196 197ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) { 198 slot_ = 0; 199 base::subtle::Release_Store(&initialized_, 0); 200 Initialize(destructor); 201} 202 203void ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) { 204 PlatformThreadLocalStorage::TLSKey key = 205 base::subtle::NoBarrier_Load(&g_native_tls_key); 206 if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES || 207 !PlatformThreadLocalStorage::GetTLSValue(key)) 208 ConstructTlsVector(); 209 210 // Grab a new slot. 211 slot_ = base::subtle::NoBarrier_AtomicIncrement(&g_last_used_tls_key, 1); 212 DCHECK_GT(slot_, 0); 213 CHECK_LT(slot_, kThreadLocalStorageSize); 214 215 // Setup our destructor. 216 g_tls_destructors[slot_] = destructor; 217 base::subtle::Release_Store(&initialized_, 1); 218} 219 220void ThreadLocalStorage::StaticSlot::Free() { 221 // At this time, we don't reclaim old indices for TLS slots. 222 // So all we need to do is wipe the destructor. 223 DCHECK_GT(slot_, 0); 224 DCHECK_LT(slot_, kThreadLocalStorageSize); 225 g_tls_destructors[slot_] = NULL; 226 slot_ = 0; 227 base::subtle::Release_Store(&initialized_, 0); 228} 229 230void* ThreadLocalStorage::StaticSlot::Get() const { 231 void** tls_data = static_cast<void**>( 232 PlatformThreadLocalStorage::GetTLSValue( 233 base::subtle::NoBarrier_Load(&g_native_tls_key))); 234 if (!tls_data) 235 tls_data = ConstructTlsVector(); 236 DCHECK_GT(slot_, 0); 237 DCHECK_LT(slot_, kThreadLocalStorageSize); 238 return tls_data[slot_]; 239} 240 241void ThreadLocalStorage::StaticSlot::Set(void* value) { 242 void** tls_data = static_cast<void**>( 243 PlatformThreadLocalStorage::GetTLSValue( 244 base::subtle::NoBarrier_Load(&g_native_tls_key))); 245 if (!tls_data) 246 tls_data = ConstructTlsVector(); 247 DCHECK_GT(slot_, 0); 248 DCHECK_LT(slot_, kThreadLocalStorageSize); 249 tls_data[slot_] = value; 250} 251 252} // namespace base 253