bool LockManager::unlock(LockRequest* request) {
invariant(request->lock);
// Fast path for decrementing multiple references of the same lock. It is safe to do this
// without locking, because 1) all calls for the same lock request must be done on the same
// thread and 2) if there are lock requests hanging of a given LockHead, then this lock
// will never disappear.
request->recursiveCount--;
if ((request->status == LockRequest::STATUS_GRANTED) && (request->recursiveCount > 0)) {
return false;
}
LockHead* lock = request->lock;
LockBucket* bucket = _getBucket(lock->resourceId);
scoped_spinlock scopedLock(bucket->mutex);
invariant(lock->grantedQueue != NULL);
invariant(lock->grantedModes != 0);
if (request->status == LockRequest::STATUS_WAITING) {
// This cancels a pending lock request
invariant(request->recursiveCount == 0);
lock->removeFromConflictQueue(request);
lock->changeConflictModeCount(request->mode, LockHead::Decrement);
}
else if (request->status == LockRequest::STATUS_CONVERTING) {
// This cancels a pending convert request
invariant(request->recursiveCount > 0);
// Lock only goes from GRANTED to CONVERTING, so cancelling the conversion request
// brings it back to the previous granted mode.
request->status = LockRequest::STATUS_GRANTED;
lock->conversionsCount--;
lock->changeGrantedModeCount(request->convertMode, LockHead::Decrement);
request->convertMode = MODE_NONE;
_onLockModeChanged(lock, lock->grantedCounts[request->convertMode] == 0);
}
else if (request->status == LockRequest::STATUS_GRANTED) {
// This releases a currently held lock and is the most common path, so it should be
// as efficient as possible.
invariant(request->recursiveCount == 0);
// Remove from the granted list
lock->removeFromGrantedQueue(request);
lock->changeGrantedModeCount(request->mode, LockHead::Decrement);
_onLockModeChanged(lock, lock->grantedCounts[request->mode] == 0);
}
else {
// Invalid request status
invariant(false);
}
return (request->recursiveCount == 0);
}
LockResult LockManager::lock(const ResourceId& resId, LockRequest* request, LockMode mode) {
dassert(mode > MODE_NONE);
// Fast path for acquiring the same lock multiple times in modes, which are already covered
// by the current mode. It is safe to do this without locking, because 1) all calls for the
// same lock request must be done on the same thread and 2) if there are lock requests
// hanging off a given LockHead, then this lock will never disappear.
if ((LockConflictsTable[request->mode] | LockConflictsTable[mode]) ==
LockConflictsTable[request->mode]) {
request->recursiveCount++;
return LOCK_OK;
}
// TODO: For the time being we do not need conversions between unrelated lock modes (i.e.,
// modes which both add and remove to the conflicts set), so these are not implemented yet
// (e.g., S -> IX).
invariant((LockConflictsTable[request->mode] | LockConflictsTable[mode]) ==
LockConflictsTable[mode]);
LockBucket* bucket = _getBucket(resId);
scoped_spinlock scopedLock(bucket->mutex);
LockHead* lock;
LockHeadMap::iterator it = bucket->data.find(resId);
if (it == bucket->data.end()) {
// Lock is free (not on the map)
invariant(request->status == LockRequest::STATUS_NEW);
lock = new LockHead(resId);
bucket->data.insert(LockHeadPair(resId, lock));
}
else {
// Lock is not free
lock = it->second;
}
// Sanity check if requests are being reused
invariant(request->lock == NULL || request->lock == lock);
request->lock = lock;
request->recursiveCount++;
if (request->status == LockRequest::STATUS_NEW) {
invariant(request->recursiveCount == 1);
// New lock request
if (conflicts(mode, lock->grantedModes)) {
request->status = LockRequest::STATUS_WAITING;
request->mode = mode;
request->convertMode = MODE_NONE;
// Put it on the conflict queue. This is the place where various policies could be
// applied for where in the wait queue does a request go.
lock->addToConflictQueue(request);
lock->changeConflictModeCount(mode, LockHead::Increment);
return LOCK_WAITING;
}
else { // No conflict, new request
request->status = LockRequest::STATUS_GRANTED;
request->mode = mode;
request->convertMode = MODE_NONE;
lock->addToGrantedQueue(request);
lock->changeGrantedModeCount(mode, LockHead::Increment);
return LOCK_OK;
}
}
else {
// If we are here, we already hold the lock in some mode. In order to keep it simple,
// we do not allow requesting a conversion while a lock is already waiting or pending
// conversion, hence the assertion below.
invariant(request->status == LockRequest::STATUS_GRANTED);
invariant(request->recursiveCount > 1);
invariant(request->mode != mode);
// Construct granted mask without our current mode, so that it is not counted as
// conflicting
uint32_t grantedModesWithoutCurrentRequest = 0;
// We start the counting at 1 below, because LockModesCount also includes MODE_NONE
// at position 0, which can never be acquired/granted.
for (uint32_t i = 1; i < LockModesCount; i++) {
const uint32_t currentRequestHolds =
(request->mode == static_cast<LockMode>(i) ? 1 : 0);
if (lock->grantedCounts[i] > currentRequestHolds) {
grantedModesWithoutCurrentRequest |= modeMask(static_cast<LockMode>(i));
}
}
// This check favours conversion requests over pending requests. For example:
//
// T1 requests lock L in IS
// T2 requests lock L in X
// T1 then upgrades L from IS -> S
//
// Because the check does not look into the conflict modes bitmap, it will grant L to
// T1 in S mode, instead of block, which would otherwise cause deadlock.
if (conflicts(mode, grantedModesWithoutCurrentRequest)) {
request->status = LockRequest::STATUS_CONVERTING;
request->convertMode = mode;
lock->conversionsCount++;
lock->changeGrantedModeCount(request->convertMode, LockHead::Increment);
return LOCK_WAITING;
}
else { // No conflict, existing request
lock->changeGrantedModeCount(mode, LockHead::Increment);
lock->changeGrantedModeCount(request->mode, LockHead::Decrement);
request->mode = mode;
return LOCK_OK;
}
}
}