diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index bae16d7fb53..cb3cc09f16d 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3739,8 +3739,8 @@ my_shmem_init(void *arg)
       startup, it will immediately call the appropriate callbacks, depending
       on whether the requested memory areas were already initialized by
       another backend. The callbacks will be called while holding an internal
-      lock, which prevents concurrent two backends from initializing the
-      memory area concurrently.
+      lock (ShmemIndexLock), which prevents the race condition of two backends
+      trying to initializing the memory area at the same time.
      </para>
     </sect3>
 
diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index f1f7cd3a4ff..1fbba9c3a4c 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -918,7 +918,10 @@ CallShmemCallbacksAfterStartup(const ShmemCallbacks *callbacks)
 		return;
 	}
 
-	/* Hold ShmemIndexLock while we allocate all the shmem entries */
+	/*
+	 * Hold ShmemIndexLock while we allocate all the shmem entries and run all
+	 * the initializers.
+	 */
 	LWLockAcquire(ShmemIndexLock, LW_EXCLUSIVE);
 
 	/*
@@ -937,7 +940,7 @@ CallShmemCallbacksAfterStartup(const ShmemCallbacks *callbacks)
 			notfound_any = true;
 	}
 	if (found_any && notfound_any)
-		elog(ERROR, "found some but not all");
+		elog(ERROR, "some of the requested shmem areas have already been initialized");
 
 	/*
 	 * Allocate or attach all the shmem areas requested by the request_fn