// Timer IDs are implemented using a free-list;
// there's a vector initialized with:
// X[i] = i + 1
// and nextFreeTimerId starts with 1.
//
// Allocating a timer ID involves taking the ID from
// X[nextFreeTimerId]
// updating nextFreeTimerId to this value and returning the old value
//
// When the timer ID is allocated, its cell in the vector is unused (it's a
// free list). As an added protection, we use the cell to store an invalid
// (negative) value that we can later check for integrity.
//
// (continues below).
int QAbstractEventDispatcherPrivate::allocateTimerId()
{
int timerId, newTimerId;
int at, *b;
do {
timerId = nextFreeTimerId; //.loadAcquire(); // ### FIXME Proper memory ordering semantics
// which bucket are we looking in?
int which = timerId & TimerIdMask;
int bucket = bucketOffset(which);
at = bucketIndex(bucket, which);
b = timerIds[bucket];
if (!b) {
// allocate a new bucket
b = allocateBucket(bucket);
if (!timerIds[bucket].testAndSetRelease(0, b)) {
// another thread won the race to allocate the bucket
delete [] b;
b = timerIds[bucket];
}
}
newTimerId = prepareNewValueWithSerialNumber(timerId, b[at]);
} while (!nextFreeTimerId.testAndSetRelaxed(timerId, newTimerId));
b[at] = -timerId;
return timerId;
}
void QAbstractEventDispatcherPrivate::releaseTimerId(int timerId)
{
int which = timerId & 0x00ffffff;
int bucket = bucketOffset(which);
int at = bucketIndex(bucket, which);
int *b = timerIds[bucket];
int freeId, newTimerId;
do {
freeId = nextFreeTimerId;
b[at] = freeId & 0x00ffffff;
newTimerId = prepareNewValueWithSerialNumber(freeId, timerId);
} while (!nextFreeTimerId.testAndSetRelease(freeId, newTimerId));
}
// Releasing a timer ID requires putting the current ID back in the vector;
// we do it by setting:
// X[timerId] = nextFreeTimerId;
// then we update nextFreeTimerId to the timer we've just released
//
// The extra code in allocateTimerId and releaseTimerId are ABA prevention
// and bucket memory. The buckets are simply to make sure we allocate only
// the necessary number of timers. See above.
//
// ABA prevention simply adds a value to 7 of the top 8 bits when resetting
// nextFreeTimerId.
void QAbstractEventDispatcherPrivate::releaseTimerId(int timerId)
{
int which = timerId & TimerIdMask;
int bucket = bucketOffset(which);
int at = bucketIndex(bucket, which);
int *b = timerIds[bucket];
Q_ASSERT_X(timerId == -b[at], "QAbstractEventDispatcher::releaseTimerId",
"Internal error: timer ID not found");
int freeId, newTimerId;
do {
freeId = nextFreeTimerId;//.loadAcquire(); // ### FIXME Proper memory ordering semantics
b[at] = freeId & TimerIdMask;
newTimerId = prepareNewValueWithSerialNumber(freeId, timerId);
} while (!nextFreeTimerId.testAndSetRelease(freeId, newTimerId));
}