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 <limits>
+
+#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;
+}
+
+
+Object* FutexEmulation::Wait(Isolate* isolate,
+                             Handle<JSArrayBuffer> array_buffer, size_t addr,
+                             int32_t value, double rel_timeout_ms) {
+  // We never want to wait longer than this amount of time; this way we can
+  // interrupt this thread even if this is an "infinitely blocking" wait.
+  // TODO(binji): How long should this be? Should it be configurable?

[Jarin, 2015/07/17 12:55:27]

No idea. Ideally, we should have some notification from the interrupt mechanism
so that you do not have to busy-wait here. Perhaps you could add a TODO here
that reminds us to fix the busy wait?

[binji, 2015/07/17 16:29:21]

Done.

I was thinking about this: because there is only one waiter per isolate, and the
waiting node is stored in the isolate, it may not be too hard to signal that
waiter from HandleInterrupts.

+  const base::TimeDelta kMaxWaitTime = base::TimeDelta::FromMilliseconds(50);
+
+  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 Smi::FromInt(Result::kNotEqual);
+  }
+
+  FutexWaitListNode* node = isolate->futex_wait_list_node();
+
+  node->backing_store_ = backing_store;
+  node->wait_addr_ = addr;
+  node->waiting_ = true;
+
+  bool use_timeout = rel_timeout_ms != V8_INFINITY;
+
+  base::TimeDelta rel_timeout;
+  if (use_timeout) {
+    // Convert to nanoseconds.
+    double rel_timeout_ns = rel_timeout_ms *
+                            base::Time::kNanosecondsPerMicrosecond *
+                            base::Time::kMicrosecondsPerMillisecond;
+    if (rel_timeout_ns >
+        static_cast<double>(std::numeric_limits<int64_t>::max())) {
+      // 2**63 nanoseconds is 292 years. Let's just treat anything greater as
+      // infinite.
+      use_timeout = false;
+    } else {
+      rel_timeout = base::TimeDelta::FromNanoseconds(
+          static_cast<int64_t>(rel_timeout_ns));
+    }
+  }
+
+  base::TimeDelta rel_time_left = rel_timeout;
+
+  wait_list_.Pointer()->AddNode(node);
+
+  Object* result;
+
+  while (true) {
+    base::TimeDelta time_to_wait = (use_timeout && rel_time_left < kMaxWaitTime)
+                                       ? rel_time_left
+                                       : kMaxWaitTime;
+
+    base::Time start_time = base::Time::NowFromSystemTime();
+    bool wait_for_result = node->cond_.WaitFor(mutex_.Pointer(), time_to_wait);
+    USE(wait_for_result);
+
+    if (!node->waiting_) {
+      result = Smi::FromInt(Result::kOk);
+      break;
+    }
+
+    // Spurious wakeup or timeout.
+    base::Time end_time = base::Time::NowFromSystemTime();
+    base::TimeDelta waited_for = end_time - start_time;
+    rel_time_left -= waited_for;
+
+    if (use_timeout && rel_time_left < base::TimeDelta::FromMicroseconds(0)) {
+      result = Smi::FromInt(Result::kTimedOut);
+      break;
+    }
+
+    // Potentially handle interrupts before continuing to wait.
+    Object* interrupt_object = isolate->stack_guard()->HandleInterrupts();
+    if (interrupt_object->IsException()) {
+      result = interrupt_object;
+      break;
+    }
+  }
+
+  wait_list_.Pointer()->RemoveNode(node);
+
+  return result;
+}
+
+
+Object* 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 Smi::FromInt(waiters_woken);
+}
+
+
+Object* 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 Smi::FromInt(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 Smi::FromInt(waiters_woken);
+}
+
+
+Object* 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 Smi::FromInt(waiters);
+}
+
+}  // namespace internal
+}  // namespace v8