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// 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 "base/synchronization/lock.h"
#include "build/build_config.h"
using base::internal::PlatformThreadLocalStorage;
// Chrome Thread Local Storage (TLS)
// This TLS system allows Chrome to use a single OS level TLS slot process-wide,
// and allows us to control the slot limits instead of being at the mercy of the
// platform. To do this, Chrome TLS replicates an array commonly found in the OS
// thread metadata.
// Overview:
// OS TLS Slots Per-Thread Per-Process Global
// ...
// [] Chrome TLS Array Chrome TLS Metadata
// [] ----------> [][][][][ ][][][][] [][][][][ ][][][][]
// [] | |
// ... V V
// Metadata Version Slot Information
// Your Data!
// Using a single OS TLS slot, Chrome TLS allocates an array on demand for the
// lifetime of each thread that requests Chrome TLS data. Each per-thread TLS
// array matches the length of the per-process global metadata array.
// A per-process global TLS metadata array tracks information about each item in
// the per-thread array:
// * Status: Tracks if the slot is allocated or free to assign.
// * Destructor: An optional destructor to call on thread destruction for that
// specific slot.
// * Version: Tracks the current version of the TLS slot. Each TLS slot
// allocation is associated with a unique version number.
// Most OS TLS APIs guarantee that a newly allocated TLS slot is
// initialized to 0 for all threads. The Chrome TLS system provides
// this guarantee by tracking the version for each TLS slot here
// on each per-thread Chrome TLS array entry. Threads that access
// a slot with a mismatched version will receive 0 as their value.
// The metadata version is incremented when the client frees a
// slot. The per-thread metadata version is updated when a client
// writes to the slot. This scheme allows for constant time
// invalidation and avoids the need to iterate through each Chrome
// TLS array to mark the slot as zero.
// Just like an OS TLS API, clients of the Chrome TLS are responsible for
// managing any necessary lifetime of the data in their slots. The only
// convenience provided is automatic destruction when a thread ends. If a client
// frees a slot, that client is responsible for destroying the data in the slot.
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 =
// The maximum number of slots in our thread local storage stack.
constexpr int kThreadLocalStorageSize = 256;
constexpr int kInvalidSlotValue = -1;
enum TlsStatus {
struct TlsMetadata {
TlsStatus status;
base::ThreadLocalStorage::TLSDestructorFunc destructor;
uint32_t version;
struct TlsVectorEntry {
void* data;
uint32_t version;
// This lock isn't needed until after we've constructed the per-thread TLS
// vector, so it's safe to use.
base::Lock* GetTLSMetadataLock() {
static auto* lock = new base::Lock();
return lock;
TlsMetadata g_tls_metadata[kThreadLocalStorageSize];
size_t g_last_assigned_slot = 0;
// The maximum number of times to try to clear slots by calling destructors.
// Use pthread naming convention for clarity.
constexpr int kMaxDestructorIterations = 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.
TlsVectorEntry* 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)
TlsVectorEntry 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.
TlsVectorEntry* tls_data = new TlsVectorEntry[kThreadLocalStorageSize];
memcpy(tls_data, stack_allocated_tls_data, sizeof(stack_allocated_tls_data));
PlatformThreadLocalStorage::SetTLSValue(key, tls_data);
return tls_data;
void OnThreadExitInternal(TlsVectorEntry* tls_data) {
// 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_metadata destructors. (i.e., don't even call
// delete[] after we're done with destructors.)
TlsVectorEntry 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.
// Snapshot the TLS Metadata so we don't have to lock on every access.
TlsMetadata tls_metadata[kThreadLocalStorageSize];
base::AutoLock auto_lock(*GetTLSMetadataLock());
memcpy(tls_metadata, g_tls_metadata, sizeof(g_tls_metadata));
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 iterate several more times, so it is really not that critical
// (but it might help).
for (int slot = 0; slot < kThreadLocalStorageSize ; ++slot) {
void* tls_value = stack_allocated_tls_data[slot].data;
if (!tls_value || tls_metadata[slot].status == TlsStatus::FREE ||
stack_allocated_tls_data[slot].version != tls_metadata[slot].version)
base::ThreadLocalStorage::TLSDestructorFunc destructor =
if (!destructor)
stack_allocated_tls_data[slot].data = nullptr; // 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, nullptr);
} // 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
void ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) {
PlatformThreadLocalStorage::TLSKey key =
if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
!PlatformThreadLocalStorage::GetTLSValue(key)) {
// Grab a new slot.
slot_ = kInvalidSlotValue;
version_ = 0;
base::AutoLock auto_lock(*GetTLSMetadataLock());
for (int i = 0; i < kThreadLocalStorageSize; ++i) {
// Tracking the last assigned slot is an attempt to find the next
// available slot within one iteration. Under normal usage, slots remain
// in use for the lifetime of the process (otherwise before we reclaimed
// slots, we would have run out of slots). This makes it highly likely the
// next slot is going to be a free slot.
size_t slot_candidate =
(g_last_assigned_slot + 1 + i) % kThreadLocalStorageSize;
if (g_tls_metadata[slot_candidate].status == TlsStatus::FREE) {
g_tls_metadata[slot_candidate].status = TlsStatus::IN_USE;
g_tls_metadata[slot_candidate].destructor = destructor;
g_last_assigned_slot = slot_candidate;
slot_ = slot_candidate;
version_ = g_tls_metadata[slot_candidate].version;
CHECK_NE(slot_, kInvalidSlotValue);
CHECK_LT(slot_, kThreadLocalStorageSize);
// Setup our destructor.
base::subtle::Release_Store(&initialized_, 1);
void ThreadLocalStorage::StaticSlot::Free() {
DCHECK_NE(slot_, kInvalidSlotValue);
DCHECK_LT(slot_, kThreadLocalStorageSize);
base::AutoLock auto_lock(*GetTLSMetadataLock());
g_tls_metadata[slot_].status = TlsStatus::FREE;
g_tls_metadata[slot_].destructor = nullptr;
slot_ = kInvalidSlotValue;
base::subtle::Release_Store(&initialized_, 0);
void* ThreadLocalStorage::StaticSlot::Get() const {
TlsVectorEntry* tls_data = static_cast<TlsVectorEntry*>(
if (!tls_data)
tls_data = ConstructTlsVector();
DCHECK_NE(slot_, kInvalidSlotValue);
DCHECK_LT(slot_, kThreadLocalStorageSize);
// Version mismatches means this slot was previously freed.
if (tls_data[slot_].version != version_)
return nullptr;
return tls_data[slot_].data;
void ThreadLocalStorage::StaticSlot::Set(void* value) {
TlsVectorEntry* tls_data = static_cast<TlsVectorEntry*>(
if (!tls_data)
tls_data = ConstructTlsVector();
DCHECK_NE(slot_, kInvalidSlotValue);
DCHECK_LT(slot_, kThreadLocalStorageSize);
tls_data[slot_].data = value;
tls_data[slot_].version = version_;
ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
ThreadLocalStorage::Slot::~Slot() {
void* ThreadLocalStorage::Slot::Get() const {
return tls_slot_.Get();
void ThreadLocalStorage::Slot::Set(void* value) {
} // namespace base