Atomics Futex API

src/futex-emulation.cc

+// Copyright 2015 the V8 project 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 "src/futex-emulation.h"
+
+#include "src/base/macros.h"
+#include "src/base/platform/time.h"
+#include "src/conversions.h"
+#include "src/handles-inl.h"
+#include "src/isolate.h"
+#include "src/list-inl.h"
+
+namespace v8 {
+namespace internal {
+
+base::LazyMutex FutexEmulation::mutex_ = LAZY_MUTEX_INITIALIZER;
+base::LazyInstance<FutexWaitList>::type FutexEmulation::wait_list_ =
+    LAZY_INSTANCE_INITIALIZER;
+
+
+FutexWaitList::FutexWaitList() : head_(nullptr), tail_(nullptr) {}
+
+
+void FutexWaitList::AddNode(FutexWaitListNode* node) {
+  DCHECK(node->prev_ == nullptr && node->next_ == nullptr);
+  if (tail_) {
+    tail_->next_ = node;
+  } else {
+    head_ = node;
+  }
+
+  node->prev_ = tail_;
+  node->next_ = nullptr;
+  tail_ = node;
+}
+
+
+void FutexWaitList::RemoveNode(FutexWaitListNode* node) {
+  if (node->prev_) {
+    node->prev_->next_ = node->next_;
+  } else {
+    head_ = node->next_;
+  }
+
+  if (node->next_) {
+    node->next_->prev_ = node->prev_;
+  } else {
+    tail_ = node->prev_;
+  }
+
+  node->prev_ = node->next_ = nullptr;
+}
+
+
+FutexEmulation::Result FutexEmulation::Wait(Isolate* isolate,
+                                            Handle<JSArrayBuffer> array_buffer,
+                                            size_t addr, int32_t value) {
+  DCHECK(addr < NumberToSize(isolate, array_buffer->byte_length()));
+
+  void* backing_store = array_buffer->backing_store();
+  int32_t* p =
+      reinterpret_cast<int32_t*>(static_cast<int8_t*>(backing_store) + addr);
+
+  base::LockGuard<base::Mutex> lock_guard(mutex_.Pointer());
+
+  if (*p != value) {
+    return Result::kNotEqual;
+  }
+
+  FutexWaitListNode* node = isolate->futex_wait_list_node();
+
+  node->backing_store_ = backing_store;
+  node->wait_addr_ = addr;
+  node->waiting_ = true;
+
+  wait_list_.Pointer()->AddNode(node);
+
+  while (node->waiting_) {
+    node->cond_.Wait(mutex_.Pointer());

[Jarin, 2015/07/15 07:42:21]

Benedikt pointed out that we cannot block the main thread indefinitely.

Either we block only for small time quanta and allow interruption from the
embedder (i.e., Blink) or we find somehow let the embedder do the waiting.

+  }
+
+  wait_list_.Pointer()->RemoveNode(node);
+
+  return Result::kOk;
+}
+
+
+FutexEmulation::Result FutexEmulation::Wait(
+    Isolate* isolate, Handle<JSArrayBuffer> array_buffer, size_t addr,
+    int32_t value, const base::TimeDelta& rel_timeout) {
+  DCHECK(addr < NumberToSize(isolate, array_buffer->byte_length()));
+
+  void* backing_store = array_buffer->backing_store();
+  int32_t* p =
+      reinterpret_cast<int32_t*>(static_cast<int8_t*>(backing_store) + addr);
+
+  base::LockGuard<base::Mutex> lock_guard(mutex_.Pointer());
+
+  if (*p != value) {
+    return Result::kNotEqual;
+  }
+
+  FutexWaitListNode* node = isolate->futex_wait_list_node();
+
+  node->backing_store_ = backing_store;
+  node->wait_addr_ = addr;
+  node->waiting_ = true;
+
+  base::TimeDelta rel_time_left = rel_timeout;
+
+  wait_list_.Pointer()->AddNode(node);
+
+  Result result = Result::kOk;
+
+  while (true) {
+    base::Time start_time = base::Time::NowFromSystemTime();
+    if (!node->cond_.WaitFor(mutex_.Pointer(), rel_time_left)) {
+      result = Result::kTimedOut;
+      break;
+    }
+
+    base::Time end_time = base::Time::NowFromSystemTime();
+
+    if (!node->waiting_) {
+      result = Result::kOk;
+      break;
+    }
+
+    // Spurious wakeup.
+    base::TimeDelta waited_for = end_time - start_time;
+    rel_time_left -= waited_for;
+  }
+
+  wait_list_.Pointer()->RemoveNode(node);
+
+  return result;
+}
+
+
+int FutexEmulation::Wake(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
+                         size_t addr, int num_waiters_to_wake) {
+  DCHECK(addr < NumberToSize(isolate, array_buffer->byte_length()));
+
+  int waiters_woken = 0;
+  void* backing_store = array_buffer->backing_store();
+
+  base::LockGuard<base::Mutex> lock_guard(mutex_.Pointer());
+  FutexWaitListNode* node = wait_list_.Pointer()->head_;
+  while (node && num_waiters_to_wake > 0) {
+    if (backing_store == node->backing_store_ && addr == node->wait_addr_) {
+      node->waiting_ = false;
+      node->cond_.NotifyOne();
+      --num_waiters_to_wake;
+      waiters_woken++;
+    }
+
+    node = node->next_;
+  }
+
+  return waiters_woken;
+}
+
+
+int FutexEmulation::WakeOrRequeue(Isolate* isolate,
+                                  Handle<JSArrayBuffer> array_buffer,
+                                  size_t addr, int num_waiters_to_wake,
+                                  int32_t value, size_t addr2) {
+  DCHECK(addr < NumberToSize(isolate, array_buffer->byte_length()));
+  DCHECK(addr2 < NumberToSize(isolate, array_buffer->byte_length()));
+
+  void* backing_store = array_buffer->backing_store();
+  int32_t* p =
+      reinterpret_cast<int32_t*>(static_cast<int8_t*>(backing_store) + addr);
+
+  base::LockGuard<base::Mutex> lock_guard(mutex_.Pointer());
+  if (*p != value) {
+    return static_cast<int>(Result::kNotEqual);
+  }
+
+  // Wake |num_waiters_to_wake|
+  int waiters_woken = 0;
+  FutexWaitListNode* node = wait_list_.Pointer()->head_;
+  while (node) {
+    if (backing_store == node->backing_store_ && addr == node->wait_addr_) {
+      if (num_waiters_to_wake > 0) {
+        node->waiting_ = false;
+        node->cond_.NotifyOne();
+        --num_waiters_to_wake;
+        waiters_woken++;
+      } else {
+        node->wait_addr_ = addr2;
+      }
+    }
+
+    node = node->next_;
+  }
+
+  return waiters_woken;
+}
+
+int FutexEmulation::NumWaitersForTesting(Isolate* isolate,
+                                         Handle<JSArrayBuffer> array_buffer,
+                                         size_t addr) {
+  DCHECK(addr < NumberToSize(isolate, array_buffer->byte_length()));
+  void* backing_store = array_buffer->backing_store();
+
+  base::LockGuard<base::Mutex> lock_guard(mutex_.Pointer());
+
+  int waiters = 0;
+  FutexWaitListNode* node = wait_list_.Pointer()->head_;
+  while (node) {
+    if (backing_store == node->backing_store_ && addr == node->wait_addr_) {
+      waiters++;
+    }
+
+    node = node->next_;
+  }
+
+  return waiters;
+}
+
+}  // namespace internal
+}  // namespace v8