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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 <atomic>
- #include "base/check_op.h"
- #include "base/compiler_specific.h"
- #include "base/memory/raw_ptr_exclusion.h"
- #include "base/notreached.h"
- #include "base/synchronization/lock.h"
- #include "build/build_config.h"
- #if BUILDFLAG(IS_MAC) && defined(ARCH_CPU_X86_64)
- #include <pthread.h>
- #include <type_traits>
- #endif
- 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.
- std::atomic<PlatformThreadLocalStorage::TLSKey> g_native_tls_key{
- PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES};
- // The OS TLS slot has the following states. The TLS slot's lower 2 bits contain
- // the state, the upper bits the TlsVectorEntry*.
- // * kUninitialized: Any call to Slot::Get()/Set() will create the base
- // per-thread TLS state. kUninitialized must be null.
- // * kInUse: value has been created and is in use.
- // * kDestroying: Set when the thread is exiting prior to deleting any of the
- // values stored in the TlsVectorEntry*. This state is necessary so that
- // sequence/task checks won't be done while in the process of deleting the
- // tls entries (see comments in SequenceCheckerImpl for more details).
- // * kDestroyed: All of the values in the vector have been deallocated and
- // the TlsVectorEntry has been deleted.
- //
- // Final States:
- // * Windows: kDestroyed. Windows does not iterate through the OS TLS to clean
- // up the values.
- // * POSIX: kUninitialized. POSIX iterates through TLS until all slots contain
- // nullptr.
- //
- // More details on this design:
- // We need some type of thread-local state to indicate that the TLS system has
- // been destroyed. To do so, we leverage the multi-pass nature of destruction
- // of pthread_key.
- //
- // a) After destruction of TLS system, we set the pthread_key to a sentinel
- // kDestroyed.
- // b) All calls to Slot::Get() DCHECK that the state is not kDestroyed, and
- // any system which might potentially invoke Slot::Get() after destruction
- // of TLS must check ThreadLocalStorage::ThreadIsBeingDestroyed().
- // c) After a full pass of the pthread_keys, on the next invocation of
- // ConstructTlsVector(), we'll then set the key to nullptr.
- // d) At this stage, the TLS system is back in its uninitialized state.
- // e) If in the second pass of destruction of pthread_keys something were to
- // re-initialize TLS [this should never happen! Since the only code which
- // uses Chrome TLS is Chrome controlled, we should really be striving for
- // single-pass destruction], then TLS will be re-initialized and then go
- // through the 2-pass destruction system again. Everything should just
- // work (TM).
- // The state of the tls-entry.
- enum class TlsVectorState {
- kUninitialized = 0,
- // In the process of destroying the entries in the vector.
- kDestroying,
- // All of the entries and the vector has been destroyed.
- kDestroyed,
- // The vector has been initialized and is in use.
- kInUse,
- kMaxValue = kInUse
- };
- // Bit-mask used to store TlsVectorState.
- constexpr uintptr_t kVectorStateBitMask = 3;
- static_assert(static_cast<int>(TlsVectorState::kMaxValue) <=
- kVectorStateBitMask,
- "number of states must fit in header");
- static_assert(static_cast<int>(TlsVectorState::kUninitialized) == 0,
- "kUninitialized must be null");
- // The maximum number of slots in our thread local storage stack.
- constexpr size_t kThreadLocalStorageSize = 256;
- enum TlsStatus {
- FREE,
- IN_USE,
- };
- struct TlsMetadata {
- TlsStatus status;
- base::ThreadLocalStorage::TLSDestructorFunc destructor;
- // Incremented every time a slot is reused. Used to detect reuse of slots.
- uint32_t version;
- };
- struct TlsVectorEntry {
- // `data` is not a raw_ptr<...> for performance reasons (based on analysis of
- // sampling profiler data and tab_search:top100:2020).
- RAW_PTR_EXCLUSION 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 size_t kMaxDestructorIterations = kThreadLocalStorageSize;
- // Sets the value and state of the vector.
- void SetTlsVectorValue(PlatformThreadLocalStorage::TLSKey key,
- TlsVectorEntry* tls_data,
- TlsVectorState state) {
- DCHECK(tls_data || (state == TlsVectorState::kUninitialized) ||
- (state == TlsVectorState::kDestroyed));
- PlatformThreadLocalStorage::SetTLSValue(
- key, reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(tls_data) |
- static_cast<uintptr_t>(state)));
- }
- // Returns the tls vector and current state from the raw tls value.
- TlsVectorState GetTlsVectorStateAndValue(void* tls_value,
- TlsVectorEntry** entry = nullptr) {
- if (entry) {
- *entry = reinterpret_cast<TlsVectorEntry*>(
- reinterpret_cast<uintptr_t>(tls_value) & ~kVectorStateBitMask);
- }
- return static_cast<TlsVectorState>(reinterpret_cast<uintptr_t>(tls_value) &
- kVectorStateBitMask);
- }
- // Returns the tls vector and state using the tls key.
- TlsVectorState GetTlsVectorStateAndValue(PlatformThreadLocalStorage::TLSKey key,
- TlsVectorEntry** entry = nullptr) {
- // Only on x86_64, the implementation is not stable on ARM64. For instance, in
- // macOS 11, the TPIDRRO_EL0 registers holds the CPU index in the low bits,
- // which is not the case in macOS 12. See libsyscall/os/tsd.h in XNU
- // (_os_tsd_get_direct() is used by pthread_getspecific() internally).
- #if BUILDFLAG(IS_MAC) && defined(ARCH_CPU_X86_64)
- // On macOS, pthread_getspecific() is in libSystem, so a call to it has to go
- // through PLT. However, and contrary to some other platforms, *all* TLS keys
- // are in a static array in the thread structure. So they are *always* at a
- // fixed offset from the segment register holding the thread structure
- // address.
- //
- // We could use _pthread_getspecific_direct(), but it is not
- // exported. However, on all macOS versions we support, the TLS array is at
- // %gs. This is used in V8 and PartitionAlloc, and can also be seen by looking
- // at pthread_getspecific() disassembly:
- //
- // libsystem_pthread.dylib`pthread_getspecific:
- // libsystem_pthread.dylib[0x7ff800316099] <+0>: movq %gs:(,%rdi,8), %rax
- // libsystem_pthread.dylib[0x7ff8003160a2] <+9>: retq
- //
- // This function is essentially inlining the content of pthread_getspecific()
- // here.
- //
- // Note that this likely ends up being even faster than thread_local for
- // typical Chromium builds where the code is in a dynamic library. For the
- // static executable case, this is likely equivalent.
- static_assert(
- std::is_same<PlatformThreadLocalStorage::TLSKey, pthread_key_t>::value,
- "The special-case below assumes that the platform TLS implementation is "
- "pthread.");
- intptr_t platform_tls_value;
- asm("movq %%gs:(,%1,8), %0;" : "=r"(platform_tls_value) : "r"(key));
- return GetTlsVectorStateAndValue(reinterpret_cast<void*>(platform_tls_value),
- entry);
- #else
- return GetTlsVectorStateAndValue(PlatformThreadLocalStorage::GetTLSValue(key),
- entry);
- #endif
- }
- // 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 =
- g_native_tls_key.load(std::memory_order_relaxed);
- 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.
- PlatformThreadLocalStorage::TLSKey old_key =
- PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES;
- if (!g_native_tls_key.compare_exchange_strong(old_key, key,
- std::memory_order_relaxed,
- std::memory_order_relaxed)) {
- // 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 = g_native_tls_key.load(std::memory_order_relaxed);
- }
- }
- CHECK_EQ(GetTlsVectorStateAndValue(key), TlsVectorState::kUninitialized);
- // 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.
- SetTlsVectorValue(key, stack_allocated_tls_data, TlsVectorState::kInUse);
- // 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));
- SetTlsVectorValue(key, tls_data, TlsVectorState::kInUse);
- return tls_data;
- }
- void OnThreadExitInternal(TlsVectorEntry* tls_data) {
- DCHECK(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 =
- g_native_tls_key.load(std::memory_order_relaxed);
- SetTlsVectorValue(key, stack_allocated_tls_data, TlsVectorState::kDestroying);
- 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));
- }
- size_t remaining_attempts = kMaxDestructorIterations + 1;
- 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 (size_t 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)
- continue;
- base::ThreadLocalStorage::TLSDestructorFunc destructor =
- tls_metadata[slot].destructor;
- if (!destructor)
- continue;
- stack_allocated_tls_data[slot].data = nullptr; // 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.
- SetTlsVectorValue(key, nullptr, TlsVectorState::kDestroyed);
- }
- } // namespace
- namespace base {
- namespace internal {
- #if BUILDFLAG(IS_WIN)
- void PlatformThreadLocalStorage::OnThreadExit() {
- PlatformThreadLocalStorage::TLSKey key =
- g_native_tls_key.load(std::memory_order_relaxed);
- if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES)
- return;
- TlsVectorEntry* tls_vector = nullptr;
- const TlsVectorState state = GetTlsVectorStateAndValue(key, &tls_vector);
- // On Windows, thread destruction callbacks are only invoked once per module,
- // so there should be no way that this could be invoked twice.
- DCHECK_NE(state, TlsVectorState::kDestroyed);
- // Maybe we have never initialized TLS for this thread.
- if (state == TlsVectorState::kUninitialized)
- return;
- OnThreadExitInternal(tls_vector);
- }
- #elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
- void PlatformThreadLocalStorage::OnThreadExit(void* value) {
- // On posix this function may be called twice. The first pass calls dtors and
- // sets state to kDestroyed. The second pass sets kDestroyed to
- // kUninitialized.
- TlsVectorEntry* tls_vector = nullptr;
- const TlsVectorState state = GetTlsVectorStateAndValue(value, &tls_vector);
- if (state == TlsVectorState::kDestroyed) {
- PlatformThreadLocalStorage::TLSKey key =
- g_native_tls_key.load(std::memory_order_relaxed);
- SetTlsVectorValue(key, nullptr, TlsVectorState::kUninitialized);
- return;
- }
- OnThreadExitInternal(tls_vector);
- }
- #endif // BUILDFLAG(IS_WIN)
- } // namespace internal
- // static
- bool ThreadLocalStorage::HasBeenDestroyed() {
- PlatformThreadLocalStorage::TLSKey key =
- g_native_tls_key.load(std::memory_order_relaxed);
- if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES)
- return false;
- const TlsVectorState state = GetTlsVectorStateAndValue(key);
- return state == TlsVectorState::kDestroying ||
- state == TlsVectorState::kDestroyed;
- }
- void ThreadLocalStorage::Slot::Initialize(TLSDestructorFunc destructor) {
- PlatformThreadLocalStorage::TLSKey key =
- g_native_tls_key.load(std::memory_order_relaxed);
- if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
- GetTlsVectorStateAndValue(key) == TlsVectorState::kUninitialized) {
- ConstructTlsVector();
- }
- // Grab a new slot.
- {
- base::AutoLock auto_lock(*GetTLSMetadataLock());
- for (size_t 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;
- DCHECK_EQ(kInvalidSlotValue, slot_);
- slot_ = slot_candidate;
- version_ = g_tls_metadata[slot_candidate].version;
- break;
- }
- }
- }
- CHECK_LT(slot_, kThreadLocalStorageSize);
- }
- void ThreadLocalStorage::Slot::Free() {
- DCHECK_LT(slot_, kThreadLocalStorageSize);
- {
- base::AutoLock auto_lock(*GetTLSMetadataLock());
- g_tls_metadata[slot_].status = TlsStatus::FREE;
- g_tls_metadata[slot_].destructor = nullptr;
- ++(g_tls_metadata[slot_].version);
- }
- slot_ = kInvalidSlotValue;
- }
- void* ThreadLocalStorage::Slot::Get() const {
- TlsVectorEntry* tls_data = nullptr;
- const TlsVectorState state = GetTlsVectorStateAndValue(
- g_native_tls_key.load(std::memory_order_relaxed), &tls_data);
- DCHECK_NE(state, TlsVectorState::kDestroyed);
- if (!tls_data)
- return nullptr;
- 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::Slot::Set(void* value) {
- TlsVectorEntry* tls_data = nullptr;
- const TlsVectorState state = GetTlsVectorStateAndValue(
- g_native_tls_key.load(std::memory_order_relaxed), &tls_data);
- DCHECK_NE(state, TlsVectorState::kDestroyed);
- if (UNLIKELY(!tls_data)) {
- if (!value)
- return;
- tls_data = ConstructTlsVector();
- }
- DCHECK_LT(slot_, kThreadLocalStorageSize);
- tls_data[slot_].data = value;
- tls_data[slot_].version = version_;
- }
- ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
- Initialize(destructor);
- }
- ThreadLocalStorage::Slot::~Slot() {
- Free();
- }
- } // namespace base
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