Checking !(fp->foo_flags & FOO_BUFFERED) means: this struct foo is not buffered.

This is why I change it to:

	-       if (!(fp->foo_flags & FOO_BUFFERED) && fp->foo_buffer == NULL) {
	+       if ((fp->foo_flags & FOO_BUFFERED) && fp->foo_buffer == NULL) {

As this makes me feel we ask: "If it's buffered, and the buffer is not
allocated".

Later, you do:

	-               if ((fp->foo_flags & FOO_BUFFERED) || fp->foo_buffer != NULL)

If we get back with FOO_BUFFERED still set, free() will be
executed no matter if the foo_buffer is NULL or not.

This is why I think we might want to have something
obvious:

	+               if ((fp->foo_flags & FOO_BUFFERED) && fp->foo_buffer == NULL)
	+                       fp->foo_buffer = tmpbuff;
	+               else

If we got back to the thread with a foo_flags with FOO_BUFFERED and
foo_buffer is still NULL, use allocated region of memory.

Otherwise (foo_flags doesn't have FOO_BUFFERED and foo_buffer isn't NULL or
both), we simply free allocated memory.

==== //depot/user/attilio/attilio_smpng/locking.9#4 - /home/wkoszek/p4/locking.9/locking.9 ====
@@ -226,14 +226,14 @@
 	char *tmpbuff;
 
 	mtx_lock(&fp->foo_mtx);
-	if (!(fp->foo_flags & FOO_BUFFERED) && fp->foo_buffer == NULL) {
+	if ((fp->foo_flags & FOO_BUFFERED) && fp->foo_buffer == NULL) {
 		mtx_unlock(&fp->foo_mtx);
 		tmpbuff = malloc(BUFSIZ, M_TEMP, M_ZERO | M_WAITOK);
 		mtx_lock(&fp->foo_mtx);
-		if ((fp->foo_flags & FOO_BUFFERED) || fp->foo_buffer != NULL)
+		if ((fp->foo_flags & FOO_BUFFERED) && fp->foo_buffer == NULL)
+			fp->foo_buffer = tmpbuff;
+		else
 			free(tmpbuff, M_TEMP);
-		else
-			fp->foo_buffer = tmpbuff;
 	}
 	mtx_unlock(&fp->foo_mtx);
 }