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/*
* Copyright (C) 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef SINGLE_STATE_QUEUE_H
#define SINGLE_STATE_QUEUE_H
// Non-blocking single element state queue, or
// Non-blocking single-reader / single-writer multi-word atomic load / store
#include <stdint.h>
#include <cutils/atomic.h>
namespace android {
template<typename T> class SingleStateQueue {
public:
class Mutator;
class Observer;
enum SSQ_STATUS {
SSQ_PENDING, /* = 0 */
SSQ_READ,
SSQ_DONE,
};
struct Shared {
// needs to be part of a union so don't define constructor or destructor
friend class Mutator;
friend class Observer;
private:
void init() { mAck = 0; mSequence = 0; }
volatile int32_t mAck;
volatile int32_t mSequence;
T mValue;
};
class Mutator {
public:
Mutator(Shared *shared)
: mSequence(0), mShared(shared)
{
// exactly one of Mutator and Observer must initialize, currently it is Observer
// shared->init();
}
// push new value onto state queue, overwriting previous value;
// returns a sequence number which can be used with ack()
int32_t push(const T& value)
{
Shared *shared = mShared;
int32_t sequence = mSequence;
sequence++;
android_atomic_acquire_store(sequence, &shared->mSequence);
shared->mValue = value;
sequence++;
android_atomic_release_store(sequence, &shared->mSequence);
mSequence = sequence;
// consider signalling a futex here, if we know that observer is waiting
return sequence;
}
// returns the status of the last state push. This may be a stale value.
//
// SSQ_PENDING, or 0, means it has not been observed
// SSQ_READ means it has been read
// SSQ_DONE means it has been acted upon, after Observer::done() is called
enum SSQ_STATUS ack() const
{
// in the case of SSQ_DONE, prevent any subtle data-races of subsequent reads
// being performed (out-of-order) before the ack read, should the caller be
// depending on sequentiality of reads.
const int32_t ack = android_atomic_acquire_load(&mShared->mAck);
return ack - mSequence & ~1 ? SSQ_PENDING /* seq differ */ :
ack & 1 ? SSQ_DONE : SSQ_READ;
}
// return true if a push with specified sequence number or later has been observed
bool ack(int32_t sequence) const
{
// this relies on 2's complement rollover to detect an ancient sequence number
return mShared->mAck - sequence >= 0;
}
private:
int32_t mSequence;
Shared * const mShared;
};
class Observer {
public:
Observer(Shared *shared)
: mSequence(0), mSeed(1), mShared(shared)
{
// exactly one of Mutator and Observer must initialize, currently it is Observer
shared->init();
}
// return true if value has changed
bool poll(T& value)
{
Shared *shared = mShared;
int32_t before = shared->mSequence;
if (before == mSequence) {
return false;
}
for (int tries = 0; ; ) {
const int MAX_TRIES = 5;
if (before & 1) {
if (++tries >= MAX_TRIES) {
return false;
}
before = shared->mSequence;
} else {
android_memory_barrier();
T temp = shared->mValue;
int32_t after = android_atomic_release_load(&shared->mSequence);
if (after == before) {
value = temp;
shared->mAck = before;
mSequence = before; // mSequence is even after poll success
return true;
}
if (++tries >= MAX_TRIES) {
return false;
}
before = after;
}
}
}
// (optional) used to indicate to the Mutator that the state that has been polled
// has also been acted upon.
void done()
{
const int32_t ack = mShared->mAck + 1;
// ensure all previous writes have been performed.
android_atomic_release_store(ack, &mShared->mAck); // mSequence is odd after "done"
}
private:
int32_t mSequence;
int mSeed; // for PRNG
Shared * const mShared;
};
#if 0
SingleStateQueue(void /*Shared*/ *shared);
/*virtual*/ ~SingleStateQueue() { }
static size_t size() { return sizeof(Shared); }
#endif
};
} // namespace android
#endif // SINGLE_STATE_QUEUE_H
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