// Copyright 2013 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/threading/thread_local_storage.h"
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#include "base/atomicops.h"
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#include "base/logging.h"
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using base::internal::PlatformThreadLocalStorage;
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namespace {
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// In order to make TLS destructors work, we need to keep around a function
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// pointer to the destructor for each slot. We keep this array of pointers in a
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// global (static) array.
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// We use the single OS-level TLS slot (giving us one pointer per thread) to
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// hold a pointer to a per-thread array (table) of slots that we allocate to
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// Chromium consumers.
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// g_native_tls_key is the one native TLS that we use. It stores our table.
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base::subtle::Atomic32 g_native_tls_key =
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PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES;
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// g_last_used_tls_key is the high-water-mark of allocated thread local storage.
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// Each allocation is an index into our g_tls_destructors[]. Each such index is
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// assigned to the instance variable slot_ in a ThreadLocalStorage::Slot
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// instance. We reserve the value slot_ == 0 to indicate that the corresponding
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// instance of ThreadLocalStorage::Slot has been freed (i.e., destructor called,
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// etc.). This reserved use of 0 is then stated as the initial value of
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// g_last_used_tls_key, so that the first issued index will be 1.
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base::subtle::Atomic32 g_last_used_tls_key = 0;
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// The maximum number of 'slots' in our thread local storage stack.
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const int kThreadLocalStorageSize = 256;
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// The maximum number of times to try to clear slots by calling destructors.
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// Use pthread naming convention for clarity.
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const int kMaxDestructorIterations = kThreadLocalStorageSize;
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// An array of destructor function pointers for the slots. If a slot has a
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// destructor, it will be stored in its corresponding entry in this array.
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// The elements are volatile to ensure that when the compiler reads the value
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// to potentially call the destructor, it does so once, and that value is tested
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// for null-ness and then used. Yes, that would be a weird de-optimization,
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// but I can imagine some register machines where it was just as easy to
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// re-fetch an array element, and I want to be sure a call to free the key
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// (i.e., null out the destructor entry) that happens on a separate thread can't
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// hurt the racy calls to the destructors on another thread.
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volatile base::ThreadLocalStorage::TLSDestructorFunc
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g_tls_destructors[kThreadLocalStorageSize];
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// This function is called to initialize our entire Chromium TLS system.
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// It may be called very early, and we need to complete most all of the setup
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// (initialization) before calling *any* memory allocator functions, which may
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// recursively depend on this initialization.
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// As a result, we use Atomics, and avoid anything (like a singleton) that might
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// require memory allocations.
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void** ConstructTlsVector() {
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PlatformThreadLocalStorage::TLSKey key =
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base::subtle::NoBarrier_Load(&g_native_tls_key);
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if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
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CHECK(PlatformThreadLocalStorage::AllocTLS(&key));
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// The TLS_KEY_OUT_OF_INDEXES is used to find out whether the key is set or
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// not in NoBarrier_CompareAndSwap, but Posix doesn't have invalid key, we
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// define an almost impossible value be it.
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// If we really get TLS_KEY_OUT_OF_INDEXES as value of key, just alloc
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// another TLS slot.
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if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
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PlatformThreadLocalStorage::TLSKey tmp = key;
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CHECK(PlatformThreadLocalStorage::AllocTLS(&key) &&
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key != PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES);
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PlatformThreadLocalStorage::FreeTLS(tmp);
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}
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// Atomically test-and-set the tls_key. If the key is
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// TLS_KEY_OUT_OF_INDEXES, go ahead and set it. Otherwise, do nothing, as
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// another thread already did our dirty work.
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if (PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES !=
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base::subtle::NoBarrier_CompareAndSwap(&g_native_tls_key,
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PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES,
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static_cast<base::subtle::Atomic32>(key))) {
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// We've been shortcut. Another thread replaced g_native_tls_key first so
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// we need to destroy our index and use the one the other thread got
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// first.
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PlatformThreadLocalStorage::FreeTLS(key);
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key = base::subtle::NoBarrier_Load(&g_native_tls_key);
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}
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}
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CHECK(!PlatformThreadLocalStorage::GetTLSValue(key));
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// Some allocators, such as TCMalloc, make use of thread local storage.
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// As a result, any attempt to call new (or malloc) will lazily cause such a
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// system to initialize, which will include registering for a TLS key. If we
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// are not careful here, then that request to create a key will call new back,
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// and we'll have an infinite loop. We avoid that as follows:
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// Use a stack allocated vector, so that we don't have dependence on our
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// allocator until our service is in place. (i.e., don't even call new until
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// after we're setup)
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void* stack_allocated_tls_data[kThreadLocalStorageSize];
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memset(stack_allocated_tls_data, 0, sizeof(stack_allocated_tls_data));
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// Ensure that any rentrant calls change the temp version.
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PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data);
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// Allocate an array to store our data.
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void** tls_data = new void*[kThreadLocalStorageSize];
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memcpy(tls_data, stack_allocated_tls_data, sizeof(stack_allocated_tls_data));
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PlatformThreadLocalStorage::SetTLSValue(key, tls_data);
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return tls_data;
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}
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void OnThreadExitInternal(void* value) {
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DCHECK(value);
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void** tls_data = static_cast<void**>(value);
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// Some allocators, such as TCMalloc, use TLS. As a result, when a thread
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// terminates, one of the destructor calls we make may be to shut down an
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// allocator. We have to be careful that after we've shutdown all of the
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// known destructors (perchance including an allocator), that we don't call
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// the allocator and cause it to resurrect itself (with no possibly destructor
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// call to follow). We handle this problem as follows:
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// Switch to using a stack allocated vector, so that we don't have dependence
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// on our allocator after we have called all g_tls_destructors. (i.e., don't
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// even call delete[] after we're done with destructors.)
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void* stack_allocated_tls_data[kThreadLocalStorageSize];
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memcpy(stack_allocated_tls_data, tls_data, sizeof(stack_allocated_tls_data));
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// Ensure that any re-entrant calls change the temp version.
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PlatformThreadLocalStorage::TLSKey key =
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base::subtle::NoBarrier_Load(&g_native_tls_key);
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PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data);
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delete[] tls_data; // Our last dependence on an allocator.
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int remaining_attempts = kMaxDestructorIterations;
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bool need_to_scan_destructors = true;
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while (need_to_scan_destructors) {
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need_to_scan_destructors = false;
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// Try to destroy the first-created-slot (which is slot 1) in our last
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// destructor call. That user was able to function, and define a slot with
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// no other services running, so perhaps it is a basic service (like an
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// allocator) and should also be destroyed last. If we get the order wrong,
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// then we'll itterate several more times, so it is really not that
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// critical (but it might help).
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base::subtle::Atomic32 last_used_tls_key =
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base::subtle::NoBarrier_Load(&g_last_used_tls_key);
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for (int slot = last_used_tls_key; slot > 0; --slot) {
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void* tls_value = stack_allocated_tls_data[slot];
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if (tls_value == NULL)
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continue;
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base::ThreadLocalStorage::TLSDestructorFunc destructor =
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g_tls_destructors[slot];
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if (destructor == NULL)
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continue;
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stack_allocated_tls_data[slot] = NULL; // pre-clear the slot.
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destructor(tls_value);
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// Any destructor might have called a different service, which then set
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// a different slot to a non-NULL value. Hence we need to check
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// the whole vector again. This is a pthread standard.
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need_to_scan_destructors = true;
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}
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if (--remaining_attempts <= 0) {
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NOTREACHED(); // Destructors might not have been called.
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break;
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}
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}
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// Remove our stack allocated vector.
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PlatformThreadLocalStorage::SetTLSValue(key, NULL);
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}
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} // namespace
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namespace base {
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namespace internal {
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#if defined(OS_WIN)
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void PlatformThreadLocalStorage::OnThreadExit() {
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PlatformThreadLocalStorage::TLSKey key =
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base::subtle::NoBarrier_Load(&g_native_tls_key);
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if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES)
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return;
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void *tls_data = GetTLSValue(key);
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// Maybe we have never initialized TLS for this thread.
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if (!tls_data)
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return;
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OnThreadExitInternal(tls_data);
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}
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#elif defined(OS_POSIX)
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void PlatformThreadLocalStorage::OnThreadExit(void* value) {
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OnThreadExitInternal(value);
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}
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#endif // defined(OS_WIN)
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} // namespace internal
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ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
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initialized_ = false;
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slot_ = 0;
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Initialize(destructor);
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}
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void ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) {
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PlatformThreadLocalStorage::TLSKey key =
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base::subtle::NoBarrier_Load(&g_native_tls_key);
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if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
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!PlatformThreadLocalStorage::GetTLSValue(key))
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ConstructTlsVector();
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// Grab a new slot.
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slot_ = base::subtle::NoBarrier_AtomicIncrement(&g_last_used_tls_key, 1);
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DCHECK_GT(slot_, 0);
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CHECK_LT(slot_, kThreadLocalStorageSize);
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// Setup our destructor.
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g_tls_destructors[slot_] = destructor;
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initialized_ = true;
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}
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void ThreadLocalStorage::StaticSlot::Free() {
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// At this time, we don't reclaim old indices for TLS slots.
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// So all we need to do is wipe the destructor.
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DCHECK_GT(slot_, 0);
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DCHECK_LT(slot_, kThreadLocalStorageSize);
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g_tls_destructors[slot_] = NULL;
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slot_ = 0;
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initialized_ = false;
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}
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void* ThreadLocalStorage::StaticSlot::Get() const {
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void** tls_data = static_cast<void**>(
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PlatformThreadLocalStorage::GetTLSValue(
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base::subtle::NoBarrier_Load(&g_native_tls_key)));
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if (!tls_data)
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tls_data = ConstructTlsVector();
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DCHECK_GT(slot_, 0);
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DCHECK_LT(slot_, kThreadLocalStorageSize);
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return tls_data[slot_];
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}
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void ThreadLocalStorage::StaticSlot::Set(void* value) {
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void** tls_data = static_cast<void**>(
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PlatformThreadLocalStorage::GetTLSValue(
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base::subtle::NoBarrier_Load(&g_native_tls_key)));
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if (!tls_data)
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tls_data = ConstructTlsVector();
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DCHECK_GT(slot_, 0);
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DCHECK_LT(slot_, kThreadLocalStorageSize);
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tls_data[slot_] = value;
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}
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} // namespace base
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