IODataQueueEntry * IOSharedDataQueue::peek()
{
- IODataQueueEntry *entry = 0;
+ IODataQueueEntry *entry = 0;
+ UInt32 headOffset;
+ UInt32 tailOffset;
- if (dataQueue && (dataQueue->head != dataQueue->tail)) {
+ if (!dataQueue) {
+ return NULL;
+ }
+
+ // Read head and tail with acquire barrier
+ // See rdar://problem/40780584 for an explanation of relaxed/acquire barriers
+ headOffset = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->head, __ATOMIC_RELAXED);
+ tailOffset = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->tail, __ATOMIC_ACQUIRE);
+
+ if (headOffset != tailOffset) {
IODataQueueEntry * head = 0;
UInt32 headSize = 0;
UInt32 headOffset = dataQueue->head;
UInt32 queueSize = getQueueSize();
-
+
if (headOffset >= queueSize) {
return NULL;
}
-
+
head = (IODataQueueEntry *)((char *)dataQueue->queue + headOffset);
headSize = head->size;
-
+
// Check if there's enough room before the end of the queue for a header.
// If there is room, check if there's enough room to hold the header and
// the data.
-
+
if ((headOffset > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
(headOffset + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize) ||
(headOffset + DATA_QUEUE_ENTRY_HEADER_SIZE > UINT32_MAX - headSize) ||
Boolean IOSharedDataQueue::enqueue(void * data, UInt32 dataSize)
{
- const UInt32 head = dataQueue->head; // volatile
- const UInt32 tail = dataQueue->tail;
+ UInt32 head;
+ UInt32 tail;
+ UInt32 newTail;
const UInt32 entrySize = dataSize + DATA_QUEUE_ENTRY_HEADER_SIZE;
IODataQueueEntry * entry;
+ // Force a single read of head and tail
+ // See rdar://problem/40780584 for an explanation of relaxed/acquire barriers
+ tail = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->tail, __ATOMIC_RELAXED);
+ head = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->head, __ATOMIC_ACQUIRE);
+
// Check for overflow of entrySize
if (dataSize > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) {
return false;
// exactly matches the available space at the end of the queue.
// The tail can range from 0 to dataQueue->queueSize inclusive.
- OSAddAtomic(entrySize, (SInt32 *)&dataQueue->tail);
+ newTail = tail + entrySize;
}
else if ( head > entrySize ) // Is there enough room at the beginning?
{
}
memcpy(&dataQueue->queue->data, data, dataSize);
- OSCompareAndSwap(dataQueue->tail, entrySize, &dataQueue->tail);
+ newTail = entrySize;
}
else
{
entry->size = dataSize;
memcpy(&entry->data, data, dataSize);
- OSAddAtomic(entrySize, (SInt32 *)&dataQueue->tail);
+ newTail = tail + entrySize;
}
else
{
return false; // queue is full
}
}
-
- // Send notification (via mach message) that data is available.
-
- if ( ( head == tail ) /* queue was empty prior to enqueue() */
- || ( dataQueue->head == tail ) ) /* queue was emptied during enqueue() */
- {
- sendDataAvailableNotification();
- }
-
- return true;
+
+ // Publish the data we just enqueued
+ __c11_atomic_store((_Atomic UInt32 *)&dataQueue->tail, newTail, __ATOMIC_RELEASE);
+
+ if (tail != head) {
+ //
+ // The memory barrier below paris with the one in ::dequeue
+ // so that either our store to the tail cannot be missed by
+ // the next dequeue attempt, or we will observe the dequeuer
+ // making the queue empty.
+ //
+ // Of course, if we already think the queue is empty,
+ // there's no point paying this extra cost.
+ //
+ __c11_atomic_thread_fence(__ATOMIC_SEQ_CST);
+ head = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->head, __ATOMIC_RELAXED);
+ }
+
+ if (tail == head) {
+ // Send notification (via mach message) that data is now available.
+ sendDataAvailableNotification();
+ }
+ return true;
}
Boolean IOSharedDataQueue::dequeue(void *data, UInt32 *dataSize)
Boolean retVal = TRUE;
IODataQueueEntry * entry = 0;
UInt32 entrySize = 0;
+ UInt32 headOffset = 0;
+ UInt32 tailOffset = 0;
UInt32 newHeadOffset = 0;
- if (dataQueue) {
- if (dataQueue->head != dataQueue->tail) {
- IODataQueueEntry * head = 0;
- UInt32 headSize = 0;
- UInt32 headOffset = dataQueue->head;
- UInt32 queueSize = getQueueSize();
-
- if (headOffset > queueSize) {
- return false;
- }
-
- head = (IODataQueueEntry *)((char *)dataQueue->queue + headOffset);
- headSize = head->size;
-
- // we wrapped around to beginning, so read from there
- // either there was not even room for the header
- if ((headOffset > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
- (headOffset + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize) ||
- // or there was room for the header, but not for the data
- (headOffset + DATA_QUEUE_ENTRY_HEADER_SIZE > UINT32_MAX - headSize) ||
- (headOffset + headSize + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize)) {
- // Note: we have to wrap to the beginning even with the UINT32_MAX checks
- // because we have to support a queueSize of UINT32_MAX.
- entry = dataQueue->queue;
- entrySize = entry->size;
- if ((entrySize > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
- (entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize)) {
- return false;
- }
- newHeadOffset = entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE;
- // else it is at the end
- } else {
- entry = head;
- entrySize = entry->size;
- if ((entrySize > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
- (entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE > UINT32_MAX - headOffset) ||
- (entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE + headOffset > queueSize)) {
- return false;
- }
- newHeadOffset = headOffset + entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE;
- }
+ if (!dataQueue || (data && !dataSize)) {
+ return false;
+ }
+
+ // Read head and tail with acquire barrier
+ // See rdar://problem/40780584 for an explanation of relaxed/acquire barriers
+ headOffset = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->head, __ATOMIC_RELAXED);
+ tailOffset = __c11_atomic_load((_Atomic UInt32 *)&dataQueue->tail, __ATOMIC_ACQUIRE);
+
+ if (headOffset != tailOffset) {
+ IODataQueueEntry * head = 0;
+ UInt32 headSize = 0;
+ UInt32 queueSize = getQueueSize();
+
+ if (headOffset > queueSize) {
+ return false;
}
-
- if (entry) {
- if (data) {
- if (dataSize) {
- if (entrySize <= *dataSize) {
- memcpy(data, &(entry->data), entrySize);
- OSCompareAndSwap( dataQueue->head, newHeadOffset, (SInt32 *)&dataQueue->head);
- } else {
- retVal = FALSE;
- }
- } else {
- retVal = FALSE;
- }
- } else {
- OSCompareAndSwap( dataQueue->head, newHeadOffset, (SInt32 *)&dataQueue->head);
- }
- if (dataSize) {
- *dataSize = entrySize;
+ head = (IODataQueueEntry *)((char *)dataQueue->queue + headOffset);
+ headSize = head->size;
+
+ // we wrapped around to beginning, so read from there
+ // either there was not even room for the header
+ if ((headOffset > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
+ (headOffset + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize) ||
+ // or there was room for the header, but not for the data
+ (headOffset + DATA_QUEUE_ENTRY_HEADER_SIZE > UINT32_MAX - headSize) ||
+ (headOffset + headSize + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize)) {
+ // Note: we have to wrap to the beginning even with the UINT32_MAX checks
+ // because we have to support a queueSize of UINT32_MAX.
+ entry = dataQueue->queue;
+ entrySize = entry->size;
+ if ((entrySize > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
+ (entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE > queueSize)) {
+ return false;
}
+ newHeadOffset = entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE;
+ // else it is at the end
} else {
- retVal = FALSE;
+ entry = head;
+ entrySize = entry->size;
+ if ((entrySize > UINT32_MAX - DATA_QUEUE_ENTRY_HEADER_SIZE) ||
+ (entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE > UINT32_MAX - headOffset) ||
+ (entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE + headOffset > queueSize)) {
+ return false;
+ }
+ newHeadOffset = headOffset + entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE;
}
- } else {
- retVal = FALSE;
- }
+ } else {
+ // empty queue
+ return false;
+ }
+
+ if (data) {
+ if (entrySize > *dataSize) {
+ // not enough space
+ return false;
+ }
+ memcpy(data, &(entry->data), entrySize);
+ *dataSize = entrySize;
+ }
+
+ __c11_atomic_store((_Atomic UInt32 *)&dataQueue->head, newHeadOffset, __ATOMIC_RELEASE);
+
+ if (newHeadOffset == tailOffset) {
+ //
+ // If we are making the queue empty, then we need to make sure
+ // that either the enqueuer notices, or we notice the enqueue
+ // that raced with our making of the queue empty.
+ //
+ __c11_atomic_thread_fence(__ATOMIC_SEQ_CST);
+ }
return retVal;
}
projects / apple / xnu.git / blobdiff