Atomics Futex API

src/futex-emulation.h

+// 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.
+
+#ifndef V8_FUTEX_EMULATION_H_
+#define V8_FUTEX_EMULATION_H_
+
+#include <stdint.h>
+
+#include "src/allocation.h"
+#include "src/base/lazy-instance.h"
+#include "src/base/macros.h"
+#include "src/base/platform/condition-variable.h"
+#include "src/base/platform/mutex.h"
+#include "src/handles.h"
+
+// Support for emulating futexes, a low-level synchronization primitive. They
+// are natively supported by Linux, but must be emulated for other platforms.
+// This library emulates them on all platforms using mutexes and condition
+// variables for consistency.
+//
+// This is used by the Futex API defined in the SharedArrayBuffer draft spec,
+// found here
+// https://docs.google.com/document/d/1NDGA_gZJ7M7w1Bh8S0AoDyEqwDdRh4uSoTPSNn77PFk
+
+namespace v8 {
+
+namespace base {
+class TimeDelta;
+}  // base
+
+namespace internal {
+
+class Isolate;
+
+class FutexWaitListNode {
+ public:
+  FutexWaitListNode()
+      : prev_(nullptr),
+        next_(nullptr),
+        backing_store_(nullptr),
+        wait_addr_(0),
+        waiting_(false) {}
+
+ private:
+  friend class FutexEmulation;
+  friend class FutexWaitList;
+
+  base::ConditionVariable cond_;
+  FutexWaitListNode* prev_;
+  FutexWaitListNode* next_;
+  void* backing_store_;
+  size_t wait_addr_;
+  bool waiting_;
+
+  DISALLOW_COPY_AND_ASSIGN(FutexWaitListNode);
+};
+
+
+class FutexWaitList {
+ public:
+  FutexWaitList();
+
+  void AddNode(FutexWaitListNode* node);
+  void RemoveNode(FutexWaitListNode* node);
+
+ private:
+  friend class FutexEmulation;
+
+  FutexWaitListNode* head_;
+  FutexWaitListNode* tail_;
+
+  DISALLOW_COPY_AND_ASSIGN(FutexWaitList);
+};
+
+
+class FutexEmulation : public AllStatic {
+ public:
+  // These must match the values in src/harmony-atomics.js
+  enum class Result {
+    kOk = 0,
+    kNotEqual = -1,
+    kTimedOut = -2,
+  };
+
+  // Check that array_buffer[addr] == value, and return kNotEqual if not. If
+  // they are equal, block execution on |isolate|'s thread until woken via
+  // |Wake|, then return kOk. The initial check and the decision to wait happen
+  // atomically.
+  static Result Wait(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
+                     size_t addr, int32_t value);
+
+  // Check that array_buffer[addr] == value, and return kNotEqual if not. If
+  // they are equal, block execution on |isolate|'s thread until woken via
+  // |Wake|, or when the time given in |rel_timeout| elapses. If woken, return
+  // kOk, otherwise return kTimedOut. The initial check and the decision to
+  // wait happen atomically.
+  static Result Wait(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
+                     size_t addr, int32_t value,
+                     const base::TimeDelta& rel_timeout);
+
+  // Wake |num_waiters_to_wake| threads that are waiting on the given |addr|.
+  // The rest of the waiters will continue to wait. The return value is the
+  // number of woken waiters.
+  static int Wake(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
+                  size_t addr, int num_waiters_to_wake);
+
+  // Check that array_buffer[addr] == value, and return kNotEqual if not. If
+  // they are equal, wake |num_waiters_to_wake| threads that are waiting on the
+  // given |addr|. The rest of the waiters will continue to wait, but will now
+  // be waiting on |addr2| instead of |addr|. The return value is the number of
+  // woken waiters or kNotEqual as described above.
+  static int WakeOrRequeue(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
+                           size_t addr, int num_waiters_to_wake, int32_t value,
+                           size_t addr2);
+
+  // Return the number of threads waiting on |addr|. Should only be used for
+  // testing.
+  static int NumWaitersForTesting(Isolate* isolate,
+                                  Handle<JSArrayBuffer> array_buffer,
+                                  size_t addr);
+
+ private:
+  static base::LazyMutex mutex_;
+  static base::LazyInstance<FutexWaitList>::type wait_list_;
+};
+}
+}  // namespace v8::internal
+
+#endif  // V8_FUTEX_EMULATION_H_