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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_EXECUTION_FUTEX_EMULATION_H_
#define V8_EXECUTION_FUTEX_EMULATION_H_
#include <stdint.h>
#include "src/base/atomicops.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/utils/allocation.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://github.com/tc39/ecmascript_sharedmem
namespace v8 {
namespace base {
class TimeDelta;
} // namespace base
namespace internal {
template <typename T>
class Handle;
class Isolate;
class JSArrayBuffer;
class AtomicsWaitWakeHandle {
public:
explicit AtomicsWaitWakeHandle(Isolate* isolate) : isolate_(isolate) {}
void Wake();
inline bool has_stopped() const { return stopped_; }
private:
Isolate* isolate_;
bool stopped_ = false;
};
class FutexWaitListNode {
public:
FutexWaitListNode()
: prev_(nullptr),
next_(nullptr),
backing_store_(nullptr),
wait_addr_(0),
waiting_(false),
interrupted_(false) {}
void NotifyWake();
private:
friend class FutexEmulation;
friend class FutexWaitList;
friend class ResetWaitingOnScopeExit;
base::ConditionVariable cond_;
// prev_ and next_ are protected by FutexEmulation::mutex_.
FutexWaitListNode* prev_;
FutexWaitListNode* next_;
void* backing_store_;
size_t wait_addr_;
// waiting_ and interrupted_ are protected by FutexEmulation::mutex_
// if this node is currently contained in FutexEmulation::wait_list_
// or an AtomicsWaitWakeHandle has access to it.
bool waiting_;
bool interrupted_;
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 ResetWaitingOnScopeExit {
public:
explicit ResetWaitingOnScopeExit(FutexWaitListNode* node) : node_(node) {}
~ResetWaitingOnScopeExit() { node_->waiting_ = false; }
private:
FutexWaitListNode* node_;
DISALLOW_COPY_AND_ASSIGN(ResetWaitingOnScopeExit);
};
class FutexEmulation : public AllStatic {
public:
// Pass to Wake() to wake all waiters.
static const uint32_t kWakeAll = UINT32_MAX;
// Check that array_buffer[addr] == value, and return "not-equal" if not. If
// 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.
// If woken, return "ok", otherwise return "timed-out". The initial check and
// the decision to wait happen atomically.
static Object WaitJs32(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
size_t addr, int32_t value, double rel_timeout_ms);
// An version of WaitJs32 for int64_t values.
static Object WaitJs64(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
size_t addr, int64_t value, double rel_timeout_ms);
// Same as WaitJs above except it returns 0 (ok), 1 (not equal) and 2 (timed
// out) as expected by Wasm.
static Object Wait32(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
size_t addr, int32_t value, double rel_timeout_ms);
// Same as Wait32 above except it checks for an int64_t value in the
// array_buffer.
static Object Wait64(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
size_t addr, int64_t value, double rel_timeout_ms);
// Wake |num_waiters_to_wake| threads that are waiting on the given |addr|.
// |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.
static Object Wake(Handle<JSArrayBuffer> array_buffer, size_t addr,
uint32_t num_waiters_to_wake);
// Return the number of threads waiting on |addr|. Should only be used for
// testing.
static Object NumWaitersForTesting(Handle<JSArrayBuffer> array_buffer,
size_t addr);
private:
friend class FutexWaitListNode;
friend class AtomicsWaitWakeHandle;
template <typename T>
static Object Wait(Isolate* isolate, Handle<JSArrayBuffer> array_buffer,
size_t addr, T value, double rel_timeout_ms);
// `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.
static base::LazyMutex mutex_;
static base::LazyInstance<FutexWaitList>::type wait_list_;
};
} // namespace internal
} // namespace v8
#endif // V8_EXECUTION_FUTEX_EMULATION_H_