Add base::ThreadLocalStorage

base/threading/thread_local_storage.cc

+// 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 =
+    PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES;
+
+// 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
+    g_tls_destructors[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.
+void** 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 !=
+        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(!PlatformThreadLocalStorage::GetTLSValue(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) {
+  DCHECK(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 =
+      base::subtle::NoBarrier_Load(&g_native_tls_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 =
+        base::subtle::NoBarrier_Load(&g_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)
+        continue;
+
+      base::ThreadLocalStorage::TLSDestructorFunc destructor =
+          g_tls_destructors[slot];
+      if (destructor == NULL)
+        continue;
+      stack_allocated_tls_data[slot] = NULL;  // 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.
+  PlatformThreadLocalStorage::SetTLSValue(key, NULL);
+}
+
+}  // 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;
+  void *tls_data = GetTLSValue(key);
+  // Maybe we have never initialized TLS for this thread.
+  if (!tls_data)
+    return;
+  OnThreadExitInternal(tls_data);
+}
+#elif defined(OS_POSIX)
+void PlatformThreadLocalStorage::OnThreadExit(void* value) {
+  OnThreadExitInternal(value);
+}
+#endif  // defined(OS_WIN)
+
+}  // namespace internal
+
+ThreadLocalStorage::Slot::Slot(TLSDestructorFunc destructor) {
+  initialized_ = false;
+  slot_ = 0;
+  Initialize(destructor);
+}
+
+bool ThreadLocalStorage::StaticSlot::Initialize(TLSDestructorFunc destructor) {
+  PlatformThreadLocalStorage::TLSKey key =
+      base::subtle::NoBarrier_Load(&g_native_tls_key);
+  if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
+      !PlatformThreadLocalStorage::GetTLSValue(key))
+    ConstructTlsVector();
+
+  // 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;
+  return 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**>(
+      PlatformThreadLocalStorage::GetTLSValue(
+          base::subtle::NoBarrier_Load(&g_native_tls_key)));
+  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**>(
+      PlatformThreadLocalStorage::GetTLSValue(
+          base::subtle::NoBarrier_Load(&g_native_tls_key)));
+  if (!tls_data)
+    tls_data = ConstructTlsVector();
+  DCHECK_GT(slot_, 0);
+  DCHECK_LT(slot_, kThreadLocalStorageSize);
+  tls_data[slot_] = value;
+}
+
+}  // namespace base