futex-emulation.h 5.1 KB
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// 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"
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#include "src/base/atomicops.h"
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#include "src/base/lazy-instance.h"
#include "src/base/macros.h"
#include "src/base/platform/condition-variable.h"
#include "src/base/platform/mutex.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,
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// found here: https://github.com/tc39/ecmascript_sharedmem
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namespace v8 {

namespace base {
class TimeDelta;
}  // base

namespace internal {

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template <typename T>
class Handle;
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class Isolate;
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class JSArrayBuffer;
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class AtomicsWaitWakeHandle {
 public:
  explicit AtomicsWaitWakeHandle(Isolate* isolate) : isolate_(isolate) {}

  void Wake();
  inline bool has_stopped() const { return stopped_; }

 private:
  Isolate* isolate_;
  bool stopped_ = false;
};

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class FutexWaitListNode {
 public:
  FutexWaitListNode()
      : prev_(nullptr),
        next_(nullptr),
        backing_store_(nullptr),
        wait_addr_(0),
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        waiting_(false),
        interrupted_(false) {}

  void NotifyWake();
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 private:
  friend class FutexEmulation;
  friend class FutexWaitList;
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  friend class ResetWaitingOnScopeExit;
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  base::ConditionVariable cond_;
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  // prev_ and next_ are protected by FutexEmulation::mutex_.
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  FutexWaitListNode* prev_;
  FutexWaitListNode* next_;
  void* backing_store_;
  size_t wait_addr_;
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  // waiting_ and interrupted_ are protected by FutexEmulation::mutex_
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  // if this node is currently contained in FutexEmulation::wait_list_
  // or an AtomicsWaitWakeHandle has access to it.
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  bool waiting_;
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  bool interrupted_;
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  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);
};

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class ResetWaitingOnScopeExit {
 public:
  explicit ResetWaitingOnScopeExit(FutexWaitListNode* node) : node_(node) {}
  ~ResetWaitingOnScopeExit() { node_->waiting_ = false; }

 private:
  FutexWaitListNode* node_;

  DISALLOW_COPY_AND_ASSIGN(ResetWaitingOnScopeExit);
};
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class FutexEmulation : public AllStatic {
 public:
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  // Pass to Wake() to wake all waiters.
  static const uint32_t kWakeAll = UINT32_MAX;

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  // Check that array_buffer[addr] == value, and return "not-equal" if not. If
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  // they are equal, block execution on |isolate|'s thread until woken via
  // |Wake|, or when the time given in |rel_timeout_ms| elapses. Note that
  // |rel_timeout_ms| can be Infinity.
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  // If woken, return "ok", otherwise return "timed-out". The initial check and
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  // the decision to wait happen atomically.
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  static Object WaitJs(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
                       size_t addr, int32_t value, double rel_timeout_ms);
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  // Same as WaitJs above except it returns 0 (ok), 1 (not equal) and 2 (timed
  // out) as expected by Wasm.
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  static Object Wait32(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
                       size_t addr, int32_t value, double rel_timeout_ms);
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  // Same as Wait32 above except it checks for an int64_t value in the
  // array_buffer.
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  static Object Wait64(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
                       size_t addr, int64_t value, double rel_timeout_ms);
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  // Wake |num_waiters_to_wake| threads that are waiting on the given |addr|.
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  // |num_waiters_to_wake| can be kWakeAll, in which case all waiters are
  // woken. The rest of the waiters will continue to wait. The return value is
  // the number of woken waiters.
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  static Object Wake(Handle<JSArrayBuffer> array_buffer, size_t addr,
                     uint32_t num_waiters_to_wake);
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  // Return the number of threads waiting on |addr|. Should only be used for
  // testing.
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  static Object NumWaitersForTesting(Handle<JSArrayBuffer> array_buffer,
                                     size_t addr);
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 private:
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  friend class FutexWaitListNode;
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  friend class AtomicsWaitWakeHandle;
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  template <typename T>
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  static Object Wait(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
                     size_t addr, T value, double rel_timeout_ms);
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  // `mutex_` protects the composition of `wait_list_` (i.e. no elements may be
  // added or removed without holding this mutex), as well as the `waiting_`
  // and `interrupted_` fields for each individual list node that is currently
  // part of the list. It must be the mutex used together with the `cond_`
  // condition variable of such nodes.
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  static base::LazyMutex mutex_;
  static base::LazyInstance<FutexWaitList>::type wait_list_;
};
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}  // namespace internal
}  // namespace v8
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#endif  // V8_FUTEX_EMULATION_H_