| // Copyright 2014 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #include "base/threading/thread_local_storage.h" |
| |
| #include "base/atomicops.h" |
| #include "base/logging.h" |
| #include "build/build_config.h" |
| |
| using base::internal::PlatformThreadLocalStorage; |
| |
| namespace { |
| // In order to make TLS destructors work, we need to keep around a function |
| // pointer to the destructor for each slot. We keep this array of pointers in a |
| // global (static) array. |
| // We use the single OS-level TLS slot (giving us one pointer per thread) to |
| // hold a pointer to a per-thread array (table) of slots that we allocate to |
| // Chromium consumers. |
| |
| // g_native_tls_key is the one native TLS that we use. It stores our table. |
| base::subtle::Atomic32 g_native_tls_key = |
| PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES; |
| |
| // g_last_used_tls_key is the high-water-mark of allocated thread local storage. |
| // Each allocation is an index into our g_tls_destructors[]. Each such index is |
| // assigned to the instance variable slot_ in a ThreadLocalStorage::Slot |
| // instance. We reserve the value slot_ == 0 to indicate that the corresponding |
| // instance of ThreadLocalStorage::Slot has been freed (i.e., destructor called, |
| // etc.). This reserved use of 0 is then stated as the initial value of |
| // g_last_used_tls_key, so that the first issued index will be 1. |
| base::subtle::Atomic32 g_last_used_tls_key = 0; |
| |
| // The maximum number of 'slots' in our thread local storage stack. |
| const int kThreadLocalStorageSize = 256; |
| |
| // The maximum number of times to try to clear slots by calling destructors. |
| // Use pthread naming convention for clarity. |
| const int kMaxDestructorIterations = kThreadLocalStorageSize; |
| |
| // An array of destructor function pointers for the slots. If a slot has a |
| // destructor, it will be stored in its corresponding entry in this array. |
| // The elements are volatile to ensure that when the compiler reads the value |
| // to potentially call the destructor, it does so once, and that value is tested |
| // for null-ness and then used. Yes, that would be a weird de-optimization, |
| // but I can imagine some register machines where it was just as easy to |
| // re-fetch an array element, and I want to be sure a call to free the key |
| // (i.e., null out the destructor entry) that happens on a separate thread can't |
| // hurt the racy calls to the destructors on another thread. |
| volatile base::ThreadLocalStorage::TLSDestructorFunc |
| g_tls_destructors[kThreadLocalStorageSize]; |
| |
| // This function is called to initialize our entire Chromium TLS system. |
| // It may be called very early, and we need to complete most all of the setup |
| // (initialization) before calling *any* memory allocator functions, which may |
| // recursively depend on this initialization. |
| // As a result, we use Atomics, and avoid anything (like a singleton) that might |
| // require memory allocations. |
| void** ConstructTlsVector() { |
| PlatformThreadLocalStorage::TLSKey key = |
| base::subtle::NoBarrier_Load(&g_native_tls_key); |
| if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) { |
| CHECK(PlatformThreadLocalStorage::AllocTLS(&key)); |
| |
| // The TLS_KEY_OUT_OF_INDEXES is used to find out whether the key is set or |
| // not in NoBarrier_CompareAndSwap, but Posix doesn't have invalid key, we |
| // define an almost impossible value be it. |
| // If we really get TLS_KEY_OUT_OF_INDEXES as value of key, just alloc |
| // another TLS slot. |
| if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) { |
| PlatformThreadLocalStorage::TLSKey tmp = key; |
| CHECK(PlatformThreadLocalStorage::AllocTLS(&key) && |
| key != PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES); |
| PlatformThreadLocalStorage::FreeTLS(tmp); |
| } |
| // Atomically test-and-set the tls_key. If the key is |
| // TLS_KEY_OUT_OF_INDEXES, go ahead and set it. Otherwise, do nothing, as |
| // another thread already did our dirty work. |
| if (PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES != |
| static_cast<PlatformThreadLocalStorage::TLSKey>( |
| base::subtle::NoBarrier_CompareAndSwap( |
| &g_native_tls_key, |
| PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES, key))) { |
| // We've been shortcut. Another thread replaced g_native_tls_key first so |
| // we need to destroy our index and use the one the other thread got |
| // first. |
| PlatformThreadLocalStorage::FreeTLS(key); |
| key = base::subtle::NoBarrier_Load(&g_native_tls_key); |
| } |
| } |
| CHECK(!PlatformThreadLocalStorage::GetTLSValue(key)); |
| |
| // Some allocators, such as TCMalloc, make use of thread local storage. |
| // As a result, any attempt to call new (or malloc) will lazily cause such a |
| // system to initialize, which will include registering for a TLS key. If we |
| // are not careful here, then that request to create a key will call new back, |
| // and we'll have an infinite loop. We avoid that as follows: |
| // Use a stack allocated vector, so that we don't have dependence on our |
| // allocator until our service is in place. (i.e., don't even call new until |
| // after we're setup) |
| void* stack_allocated_tls_data[kThreadLocalStorageSize]; |
| memset(stack_allocated_tls_data, 0, sizeof(stack_allocated_tls_data)); |
| // Ensure that any rentrant calls change the temp version. |
| PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data); |
| |
| // Allocate an array to store our data. |
| void** tls_data = new void*[kThreadLocalStorageSize]; |
| memcpy(tls_data, stack_allocated_tls_data, sizeof(stack_allocated_tls_data)); |
| PlatformThreadLocalStorage::SetTLSValue(key, tls_data); |
| return tls_data; |
| } |
| |
| void OnThreadExitInternal(void* value) { |
| DCHECK(value); |
| void** tls_data = static_cast<void**>(value); |
| // Some allocators, such as TCMalloc, use TLS. As a result, when a thread |
| // terminates, one of the destructor calls we make may be to shut down an |
| // allocator. We have to be careful that after we've shutdown all of the |
| // known destructors (perchance including an allocator), that we don't call |
| // the allocator and cause it to resurrect itself (with no possibly destructor |
| // call to follow). We handle this problem as follows: |
| // Switch to using a stack allocated vector, so that we don't have dependence |
| // on our allocator after we have called all g_tls_destructors. (i.e., don't |
| // even call delete[] after we're done with destructors.) |
| void* stack_allocated_tls_data[kThreadLocalStorageSize]; |
| memcpy(stack_allocated_tls_data, tls_data, sizeof(stack_allocated_tls_data)); |
| // Ensure that any re-entrant calls change the temp version. |
| PlatformThreadLocalStorage::TLSKey key = |
| base::subtle::NoBarrier_Load(&g_native_tls_key); |
| PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data); |
| delete[] tls_data; // Our last dependence on an allocator. |
| |
| int remaining_attempts = kMaxDestructorIterations; |
| bool need_to_scan_destructors = true; |
| while (need_to_scan_destructors) { |
| need_to_scan_destructors = false; |
| // Try to destroy the first-created-slot (which is slot 1) in our last |
| // destructor call. That user was able to function, and define a slot with |
| // no other services running, so perhaps it is a basic service (like an |
| // allocator) and should also be destroyed last. If we get the order wrong, |
| // then we'll itterate several more times, so it is really not that |
| // critical (but it might help). |
| base::subtle::Atomic32 last_used_tls_key = |
| base::subtle::NoBarrier_Load(&g_last_used_tls_key); |
| for (int slot = last_used_tls_key; slot > 0; --slot) { |
| void* tls_value = stack_allocated_tls_data[slot]; |
| if (tls_value == NULL) |
| continue; |
| |
| base::ThreadLocalStorage::TLSDestructorFunc destructor = |
| g_tls_destructors[slot]; |
| if (destructor == NULL) |
| continue; |
| stack_allocated_tls_data[slot] = NULL; // pre-clear the slot. |
| destructor(tls_value); |
| // Any destructor might have called a different service, which then set |
| // a different slot to a non-NULL value. Hence we need to check |
| // the whole vector again. This is a pthread standard. |
| need_to_scan_destructors = true; |
| } |
| if (--remaining_attempts <= 0) { |
| NOTREACHED(); // Destructors might not have been called. |
| break; |
| } |
| } |
| |
| // Remove our stack allocated vector. |
| PlatformThreadLocalStorage::SetTLSValue(key, NULL); |
| } |
| |
| } // namespace |
| |
| namespace base { |
| |
| namespace internal { |
| |
| #if defined(OS_WIN) |
| void PlatformThreadLocalStorage::OnThreadExit() { |
| PlatformThreadLocalStorage::TLSKey key = |
| base::subtle::NoBarrier_Load(&g_native_tls_key); |
| if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) |
| return; |
| void *tls_data = GetTLSValue(key); |
| // Maybe we have never initialized TLS for this thread. |
| if (!tls_data) |
| return; |
| OnThreadExitInternal(tls_data); |
| } |
| #elif defined(OS_POSIX) |
| void PlatformThreadLocalStorage::OnThreadExit(void* value) { |
| OnThreadExitInternal(value); |
| } |
| #endif // defined(OS_WIN) |
| |
| } // namespace internal |
| |
| ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) { |
| slot_ = 0; |
| base::subtle::Release_Store(&initialized_, 0); |
| Initialize(destructor); |
| } |
| |
| void ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) { |
| PlatformThreadLocalStorage::TLSKey key = |
| base::subtle::NoBarrier_Load(&g_native_tls_key); |
| if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES || |
| !PlatformThreadLocalStorage::GetTLSValue(key)) |
| ConstructTlsVector(); |
| |
| // Grab a new slot. |
| slot_ = base::subtle::NoBarrier_AtomicIncrement(&g_last_used_tls_key, 1); |
| DCHECK_GT(slot_, 0); |
| CHECK_LT(slot_, kThreadLocalStorageSize); |
| |
| // Setup our destructor. |
| g_tls_destructors[slot_] = destructor; |
| base::subtle::Release_Store(&initialized_, 1); |
| } |
| |
| void ThreadLocalStorage::StaticSlot::Free() { |
| // At this time, we don't reclaim old indices for TLS slots. |
| // So all we need to do is wipe the destructor. |
| DCHECK_GT(slot_, 0); |
| DCHECK_LT(slot_, kThreadLocalStorageSize); |
| g_tls_destructors[slot_] = NULL; |
| slot_ = 0; |
| base::subtle::Release_Store(&initialized_, 0); |
| } |
| |
| void* ThreadLocalStorage::StaticSlot::Get() const { |
| void** tls_data = static_cast<void**>( |
| PlatformThreadLocalStorage::GetTLSValue( |
| base::subtle::NoBarrier_Load(&g_native_tls_key))); |
| if (!tls_data) |
| tls_data = ConstructTlsVector(); |
| DCHECK_GT(slot_, 0); |
| DCHECK_LT(slot_, kThreadLocalStorageSize); |
| return tls_data[slot_]; |
| } |
| |
| void ThreadLocalStorage::StaticSlot::Set(void* value) { |
| void** tls_data = static_cast<void**>( |
| PlatformThreadLocalStorage::GetTLSValue( |
| base::subtle::NoBarrier_Load(&g_native_tls_key))); |
| if (!tls_data) |
| tls_data = ConstructTlsVector(); |
| DCHECK_GT(slot_, 0); |
| DCHECK_LT(slot_, kThreadLocalStorageSize); |
| tls_data[slot_] = value; |
| } |
| |
| } // namespace base |