base/threading/thread_local_storage.cc - chromium/src - Git at Google

 // 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/compiler_specific.h"
 #include "base/logging.h"
 #include "base/no_destructor.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 =
     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 int 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 {
   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;

 // 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) {
   return GetTlsVectorStateAndValue(PlatformThreadLocalStorage::GetTLSValue(key),
                                    entry);
 }

 // 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 =
       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_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 =
       base::subtle::NoBarrier_Load(&g_native_tls_key);
   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));
   }

   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)
         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 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;
   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 defined(OS_POSIX) || defined(OS_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 =
         base::subtle::NoBarrier_Load(&g_native_tls_key);
     SetTlsVectorValue(key, nullptr, TlsVectorState::kUninitialized);
     return;
   }

   OnThreadExitInternal(tls_vector);
 }
 #endif  // defined(OS_WIN)

 }  // namespace internal

 // static
 bool ThreadLocalStorage::HasBeenDestroyed() {
   PlatformThreadLocalStorage::TLSKey key =
       base::subtle::NoBarrier_Load(&g_native_tls_key);
   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 =
       base::subtle::NoBarrier_Load(&g_native_tls_key);
   if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
       GetTlsVectorStateAndValue(key) == TlsVectorState::kUninitialized) {
     ConstructTlsVector();
   }

   // Grab a new slot.
   {
     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;
         DCHECK_EQ(kInvalidSlotValue, slot_);
         slot_ = slot_candidate;
         version_ = g_tls_metadata[slot_candidate].version;
         break;
       }
     }
   }
   CHECK_NE(slot_, kInvalidSlotValue);
   CHECK_LT(slot_, kThreadLocalStorageSize);
 }

 void ThreadLocalStorage::Slot::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;
     ++(g_tls_metadata[slot_].version);
   }
   slot_ = kInvalidSlotValue;
 }

 void* ThreadLocalStorage::Slot::Get() const {
   TlsVectorEntry* tls_data = nullptr;
   const TlsVectorState state = GetTlsVectorStateAndValue(
       base::subtle::NoBarrier_Load(&g_native_tls_key), &tls_data);
   DCHECK_NE(state, TlsVectorState::kDestroyed);
   if (!tls_data)
     return nullptr;
   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::Slot::Set(void* value) {
   TlsVectorEntry* tls_data = nullptr;
   const TlsVectorState state = GetTlsVectorStateAndValue(
       base::subtle::NoBarrier_Load(&g_native_tls_key), &tls_data);
   DCHECK_NE(state, TlsVectorState::kDestroyed);
   if (!tls_data) {
     if (!value)
       return;
     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) {
   Initialize(destructor);
 }

 ThreadLocalStorage::Slot::~Slot() {
   Free();
 }

 }  // namespace base
	// 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/compiler_specific.h"
	#include "base/logging.h"
	#include "base/no_destructor.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 =
	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 int 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 {
	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;

	// 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) {
	return GetTlsVectorStateAndValue(PlatformThreadLocalStorage::GetTLSValue(key),
	entry);
	}

	// 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 =
	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_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 =
	base::subtle::NoBarrier_Load(&g_native_tls_key);
	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));
	}

	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)
	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 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;
	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 defined(OS_POSIX) \|\| defined(OS_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 =
	base::subtle::NoBarrier_Load(&g_native_tls_key);
	SetTlsVectorValue(key, nullptr, TlsVectorState::kUninitialized);
	return;
	}

	OnThreadExitInternal(tls_vector);
	}
	#endif // defined(OS_WIN)

	} // namespace internal

	// static
	bool ThreadLocalStorage::HasBeenDestroyed() {
	PlatformThreadLocalStorage::TLSKey key =
	base::subtle::NoBarrier_Load(&g_native_tls_key);
	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 =
	base::subtle::NoBarrier_Load(&g_native_tls_key);
	if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES \|\|
	GetTlsVectorStateAndValue(key) == TlsVectorState::kUninitialized) {
	ConstructTlsVector();
	}

	// Grab a new slot.
	{
	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;
	DCHECK_EQ(kInvalidSlotValue, slot_);
	slot_ = slot_candidate;
	version_ = g_tls_metadata[slot_candidate].version;
	break;
	}
	}
	}
	CHECK_NE(slot_, kInvalidSlotValue);
	CHECK_LT(slot_, kThreadLocalStorageSize);
	}

	void ThreadLocalStorage::Slot::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;
	++(g_tls_metadata[slot_].version);
	}
	slot_ = kInvalidSlotValue;
	}

	void* ThreadLocalStorage::Slot::Get() const {
	TlsVectorEntry* tls_data = nullptr;
	const TlsVectorState state = GetTlsVectorStateAndValue(
	base::subtle::NoBarrier_Load(&g_native_tls_key), &tls_data);
	DCHECK_NE(state, TlsVectorState::kDestroyed);
	if (!tls_data)
	return nullptr;
	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::Slot::Set(void* value) {
	TlsVectorEntry* tls_data = nullptr;
	const TlsVectorState state = GetTlsVectorStateAndValue(
	base::subtle::NoBarrier_Load(&g_native_tls_key), &tls_data);
	DCHECK_NE(state, TlsVectorState::kDestroyed);
	if (!tls_data) {
	if (!value)
	return;
	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) {
	Initialize(destructor);
	}

	ThreadLocalStorage::Slot::~Slot() {
	Free();
	}

	} // namespace base