xnu-4903.231.4.tar.gz

[apple/xnu.git] / iokit / Kernel / IOSharedDataQueue.cpp

diff --git a/iokit/Kernel/IOSharedDataQueue.cpp b/iokit/Kernel/IOSharedDataQueue.cpp
index 17656644ad104e1882528a49dc78cda2b0ac7b5d..385393f65c1bb7d34538870f0391859f6264d773 100644 (file)
--- a/iokit/Kernel/IOSharedDataQueue.cpp
+++ b/iokit/Kernel/IOSharedDataQueue.cpp
@@ -285,18 +285,28 @@ Boolean IOSharedDataQueue::enqueue(void * data, UInt32 dataSize)
         }
     }
 
-    // Update tail with release barrier
-    __c11_atomic_store((_Atomic UInt32 *)&dataQueue->tail, newTail, __ATOMIC_RELEASE);
-    
-    // Send notification (via mach message) that data is available.
-    
-    if ( ( tail == head )                                                   /* queue was empty prior to enqueue() */
-      || ( tail == __c11_atomic_load((_Atomic UInt32 *)&dataQueue->head, __ATOMIC_ACQUIRE) ) )   /* 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)
@@ -308,7 +318,7 @@ Boolean IOSharedDataQueue::dequeue(void *data, UInt32 *dataSize)
     UInt32              tailOffset      = 0;
     UInt32              newHeadOffset   = 0;
 
-    if (!dataQueue) {
+       if (!dataQueue || (data && !dataSize)) {
         return false;
     }
 
@@ -356,30 +366,30 @@ Boolean IOSharedDataQueue::dequeue(void *data, UInt32 *dataSize)
             }
             newHeadOffset   = headOffset + entrySize + DATA_QUEUE_ENTRY_HEADER_SIZE;
         }
-    }
-
-    if (entry) {
-        if (data) {
-            if (dataSize) {
-                if (entrySize <= *dataSize) {
-                    memcpy(data, &(entry->data), entrySize);
-                    __c11_atomic_store((_Atomic UInt32 *)&dataQueue->head, newHeadOffset, __ATOMIC_RELEASE);
-                } else {
-                    retVal = FALSE;
-                }
-            } else {
-                retVal = FALSE;
-            }
-        } else {
-            __c11_atomic_store((_Atomic UInt32 *)&dataQueue->head, newHeadOffset, __ATOMIC_RELEASE);
-        }
-
-        if (dataSize) {
-            *dataSize = entrySize;
-        }
-    } 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;
 }