Update from chromium https://crrev.com/301315

base/memory/discardable_shared_memory.cc

+// 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/memory/discardable_shared_memory.h"
+
+#if defined(OS_POSIX)
+#include <unistd.h>
+#endif
+
+#include <algorithm>
+
+#include "base/atomicops.h"
+#include "base/logging.h"
+#include "base/numerics/safe_math.h"
+
+namespace base {
+namespace {
+
+// Use a machine-sized pointer as atomic type. It will use the Atomic32 or
+// Atomic64 routines, depending on the architecture.
+typedef intptr_t AtomicType;
+typedef uintptr_t UAtomicType;
+
+// Template specialization for timestamp serialization/deserialization. This
+// is used to serialize timestamps using Unix time on systems where AtomicType
+// does not have enough precision to contain a timestamp in the standard
+// serialized format.
+template <int>
+Time TimeFromWireFormat(int64 value);
+template <int>
+int64 TimeToWireFormat(Time time);
+
+// Serialize to Unix time when using 4-byte wire format.
+// Note: 19 January 2038, this will cease to work.
+template <>
+Time ALLOW_UNUSED_TYPE TimeFromWireFormat<4>(int64 value) {
+  return value ? Time::UnixEpoch() + TimeDelta::FromSeconds(value) : Time();
+}
+template <>
+int64 ALLOW_UNUSED_TYPE TimeToWireFormat<4>(Time time) {
+  return time > Time::UnixEpoch() ? (time - Time::UnixEpoch()).InSeconds() : 0;
+}
+
+// Standard serialization format when using 8-byte wire format.
+template <>
+Time ALLOW_UNUSED_TYPE TimeFromWireFormat<8>(int64 value) {
+  return Time::FromInternalValue(value);
+}
+template <>
+int64 ALLOW_UNUSED_TYPE TimeToWireFormat<8>(Time time) {
+  return time.ToInternalValue();
+}
+
+struct SharedState {
+  enum LockState { UNLOCKED = 0, LOCKED = 1 };
+
+  explicit SharedState(AtomicType ivalue) { value.i = ivalue; }
+  SharedState(LockState lock_state, Time timestamp) {
+    int64 wire_timestamp = TimeToWireFormat<sizeof(AtomicType)>(timestamp);
+    DCHECK_GE(wire_timestamp, 0);
+    DCHECK((lock_state & ~1) == 0);
+    value.u = (static_cast<UAtomicType>(wire_timestamp) << 1) | lock_state;
+  }
+
+  LockState GetLockState() const { return static_cast<LockState>(value.u & 1); }
+
+  Time GetTimestamp() const {
+    return TimeFromWireFormat<sizeof(AtomicType)>(value.u >> 1);
+  }
+
+  // Bit 1: Lock state. Bit is set when locked.
+  // Bit 2..sizeof(AtomicType)*8: Usage timestamp. NULL time when locked or
+  // purged.
+  union {
+    AtomicType i;
+    UAtomicType u;
+  } value;
+};
+
+// Shared state is stored at offset 0 in shared memory segments.
+SharedState* SharedStateFromSharedMemory(const SharedMemory& shared_memory) {
+  DCHECK(shared_memory.memory());
+  return static_cast<SharedState*>(shared_memory.memory());
+}
+
+}  // namespace
+
+DiscardableSharedMemory::DiscardableSharedMemory() {
+}
+
+DiscardableSharedMemory::DiscardableSharedMemory(
+    SharedMemoryHandle shared_memory_handle)
+    : shared_memory_(shared_memory_handle, false) {
+}
+
+DiscardableSharedMemory::~DiscardableSharedMemory() {
+}
+
+bool DiscardableSharedMemory::CreateAndMap(size_t size) {
+  CheckedNumeric<size_t> checked_size = size;
+  checked_size += sizeof(SharedState);
+  if (!checked_size.IsValid())
+    return false;
+
+  if (!shared_memory_.CreateAndMapAnonymous(checked_size.ValueOrDie()))
+    return false;
+
+  DCHECK(last_known_usage_.is_null());
+  SharedState new_state(SharedState::LOCKED, Time());
+  subtle::Release_Store(&SharedStateFromSharedMemory(shared_memory_)->value.i,
+                        new_state.value.i);
+  return true;
+}
+
+bool DiscardableSharedMemory::Map(size_t size) {
+  return shared_memory_.Map(sizeof(SharedState) + size);
+}
+
+bool DiscardableSharedMemory::Lock() {
+  DCHECK(shared_memory_.memory());
+
+  // Return false when instance has been purged or not initialized properly by
+  // checking if |last_known_usage_| is NULL.
+  if (last_known_usage_.is_null())
+    return false;
+
+  SharedState old_state(SharedState::UNLOCKED, last_known_usage_);
+  SharedState new_state(SharedState::LOCKED, Time());
+  SharedState result(subtle::Acquire_CompareAndSwap(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i,
+      old_state.value.i,
+      new_state.value.i));
+  if (result.value.u == old_state.value.u)
+    return true;
+
+  // Update |last_known_usage_| in case the above CAS failed because of
+  // an incorrect timestamp.
+  last_known_usage_ = result.GetTimestamp();
+  return false;
+}
+
+void DiscardableSharedMemory::Unlock() {
+  DCHECK(shared_memory_.memory());
+
+  Time current_time = Now();
+  DCHECK(!current_time.is_null());
+
+  SharedState old_state(SharedState::LOCKED, Time());
+  SharedState new_state(SharedState::UNLOCKED, current_time);
+  // Note: timestamp cannot be NULL as that is a unique value used when
+  // locked or purged.
+  DCHECK(!new_state.GetTimestamp().is_null());
+  // Timestamps precision should at least be accurate to the second.
+  DCHECK_EQ((new_state.GetTimestamp() - Time::UnixEpoch()).InSeconds(),
+            (current_time - Time::UnixEpoch()).InSeconds());
+  SharedState result(subtle::Release_CompareAndSwap(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i,
+      old_state.value.i,
+      new_state.value.i));
+
+  DCHECK_EQ(old_state.value.u, result.value.u);
+
+  last_known_usage_ = current_time;
+}
+
+void* DiscardableSharedMemory::memory() const {
+  return SharedStateFromSharedMemory(shared_memory_) + 1;
+}
+
+bool DiscardableSharedMemory::Purge(Time current_time) {
+  // Early out if not mapped. This can happen if the segment was previously
+  // unmapped using a call to Close().
+  if (!shared_memory_.memory())
+    return true;
+
+  SharedState old_state(SharedState::UNLOCKED, last_known_usage_);
+  SharedState new_state(SharedState::UNLOCKED, Time());
+  SharedState result(subtle::Acquire_CompareAndSwap(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i,
+      old_state.value.i,
+      new_state.value.i));
+
+  // Update |last_known_usage_| to |current_time| if the memory is locked. This
+  // allows the caller to determine if purging failed because last known usage
+  // was incorrect or memory was locked. In the second case, the caller should
+  // most likely wait for some amount of time before attempting to purge the
+  // the memory again.
+  if (result.value.u != old_state.value.u) {
+    last_known_usage_ = result.GetLockState() == SharedState::LOCKED
+                            ? current_time
+                            : result.GetTimestamp();
+    return false;
+  }
+
+  last_known_usage_ = Time();
+  return true;
+}
+
+bool DiscardableSharedMemory::PurgeAndTruncate(Time current_time) {
+  if (!Purge(current_time))
+    return false;
+
+#if defined(OS_POSIX)
+  // Truncate shared memory to size of SharedState.
+  SharedMemoryHandle handle = shared_memory_.handle();
+  if (SharedMemory::IsHandleValid(handle)) {
+    if (HANDLE_EINTR(ftruncate(handle.fd, sizeof(SharedState))) != 0)
+      DPLOG(ERROR) << "ftruncate() failed";
+  }
+#endif
+
+  return true;
+}
+
+bool DiscardableSharedMemory::IsMemoryResident() const {
+  DCHECK(shared_memory_.memory());
+
+  SharedState result(subtle::NoBarrier_Load(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i));
+
+  return result.GetLockState() == SharedState::LOCKED ||
+         !result.GetTimestamp().is_null();
+}
+
+void DiscardableSharedMemory::Close() {
+  shared_memory_.Close();
+}
+
+Time DiscardableSharedMemory::Now() const {
+  return Time::Now();
+}
+
+}  // namespace base