blob: 20e2cb7875b092de854114490bdb948c9079a534 [file] [log] [blame]
// Copyright 2013 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"
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 =
// 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
// 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 =
if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES) {
// 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);
// 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 !=
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.
key = base::subtle::NoBarrier_Load(&g_native_tls_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) {
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 =
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 =
for (int slot = last_used_tls_key; slot > 0; --slot) {
void* tls_value = stack_allocated_tls_data[slot];
if (tls_value == NULL)
base::ThreadLocalStorage::TLSDestructorFunc destructor =
if (destructor == NULL)
stack_allocated_tls_data[slot] = NULL; // pre-clear the slot.
// 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.
// Remove our stack allocated vector.
PlatformThreadLocalStorage::SetTLSValue(key, NULL);
} // namespace
namespace base {
namespace internal {
#if defined(OS_WIN)
void PlatformThreadLocalStorage::OnThreadExit() {
PlatformThreadLocalStorage::TLSKey key =
if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES)
void *tls_data = GetTLSValue(key);
// Maybe we have never initialized TLS for this thread.
if (!tls_data)
#elif defined(OS_POSIX)
void PlatformThreadLocalStorage::OnThreadExit(void* value) {
#endif // defined(OS_WIN)
} // namespace internal
ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
initialized_ = false;
slot_ = 0;
void ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) {
PlatformThreadLocalStorage::TLSKey key =
if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
// 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;
initialized_ = true;
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;
initialized_ = false;
void* ThreadLocalStorage::StaticSlot::Get() const {
void** tls_data = static_cast<void**>(
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**>(
if (!tls_data)
tls_data = ConstructTlsVector();
DCHECK_GT(slot_, 0);
DCHECK_LT(slot_, kThreadLocalStorageSize);
tls_data[slot_] = value;
} // namespace base