diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index 26d7b7c..2d298a3 100644
--- a/src/backend/storage/ipc/shm_mq.c
+++ b/src/backend/storage/ipc/shm_mq.c
@@ -25,6 +25,8 @@
 #include "storage/spin.h"
 
 /*
+ * This structure represents the actual queue, stored in shared memory.
+ *
  * Some notes on synchronization:
  *
  * mq_receiver and mq_bytes_read can only be changed by the receiver; and
@@ -77,6 +79,8 @@ struct shm_mq
 };
 
 /*
+ * This structure is a backend-private handle for access to a queue.
+ *
  * mqh_queue is a pointer to the queue we've attached, and mqh_segment is
  * a pointer to the dynamic shared memory segment that contains it.
  *
@@ -147,10 +151,10 @@ static shm_mq_result shm_mq_notify_receiver(volatile shm_mq *mq);
 static void shm_mq_detach_callback(dsm_segment *seg, Datum arg);
 
 /* Minimum queue size is enough for header and at least one chunk of data. */
-Size shm_mq_minimum_size =
+const Size shm_mq_minimum_size =
 	MAXALIGN(offsetof(shm_mq, mq_ring)) + MAXIMUM_ALIGNOF;
 
-#define MQH_BUFSIZE				8192
+#define MQH_INITIAL_BUFSIZE				8192
 
 /*
  * Initialize a new shared message queue.
@@ -168,6 +172,7 @@ shm_mq_create(void *address, Size size)
 	Assert(size > data_offset);
 
 	/* Initialize queue header. */
+	SpinLockInit(&mq->mq_mutex);
 	mq->mq_receiver = NULL;
 	mq->mq_sender = NULL;
 	mq->mq_bytes_read = 0;
@@ -255,6 +260,8 @@ shm_mq_get_sender(shm_mq *mq)
  *
  * The memory context in effect at the time this function is called should
  * be one which will last for at least as long as the message queue itself.
+ * We'll allocate the handle in that context, and future allocations that
+ * are needed to buffer incoming data will happen in that context as well.
  *
  * If seg != NULL, the queue will be automatically detached when that dynamic
  * shared memory segment is detached.
@@ -302,7 +309,9 @@ shm_mq_attach(shm_mq *mq, dsm_segment *seg, BackgroundWorkerHandle *handle)
  * When nowait = true, we do not manipulate the state of the process latch;
  * instead, if the buffer becomes full, we return SHM_MQ_WOULD_BLOCK.  In
  * this case, the caller should call this function again, with the same
- * arguments, each time the process latch is set.
+ * arguments, each time the process latch is set.  (Once begun, the sending
+ * of a message cannot be aborted except by detaching from the queue; changing
+ * the length or payload will corrupt the queue.)
  */
 shm_mq_result
 shm_mq_send(shm_mq_handle *mqh, uint64 nbytes, void *data, bool nowait)
@@ -468,7 +477,7 @@ shm_mq_receive(shm_mq_handle *mqh, uint64 *nbytesp, void **datap, bool nowait)
 		 */
 		if (mqh->mqh_buflen < nbytes)
 		{
-			uint64		newbuflen = Max(mqh->mqh_buflen, MQH_BUFSIZE);
+			uint64		newbuflen = Max(mqh->mqh_buflen, MQH_INITIAL_BUFSIZE);
 
 			while (newbuflen < nbytes)
 				newbuflen *= 2;
@@ -856,7 +865,7 @@ shm_mq_get_bytes_read(volatile shm_mq *mq, bool *detached)
 	*detached = mq->mq_detached;
 	SpinLockRelease(&mq->mq_mutex);
 
-	return mq->mq_bytes_read;
+	return v;
 }
 
 /*
@@ -891,7 +900,7 @@ shm_mq_get_bytes_written(volatile shm_mq *mq, bool *detached)
 	*detached = mq->mq_detached;
 	SpinLockRelease(&mq->mq_mutex);
 
-	return mq->mq_bytes_written;
+	return v;
 }
 
 /*
diff --git a/src/include/storage/shm_mq.h b/src/include/storage/shm_mq.h
index 749ab3b..1ce88a1 100644
--- a/src/include/storage/shm_mq.h
+++ b/src/include/storage/shm_mq.h
@@ -65,6 +65,6 @@ extern shm_mq_result shm_mq_receive(shm_mq_handle *mqh,
 extern shm_mq_result shm_mq_wait_for_attach(shm_mq_handle *mqh);
 
 /* Smallest possible queue. */
-extern Size shm_mq_minimum_size;
+extern const Size shm_mq_minimum_size;
 
 #endif   /* SHM_MQ_H */