remark that the queue is blocking

papers/icfp2021/queue-impl.tex

@@ -292,3 +292,20 @@ Possible situations are:
 \end{itemize}%
 
 Hence competition happens only between enqueuers, or between dequeuers.
+
+A weakness of this implementation, however, is that it is blocking:
+if an enqueuer or a dequeuer halts after it has been attributed a rank but before
+it updates the corresponding slot, then after some time, any other thread trying
+to enqueue or dequeue will fail.
+%
+%Specifically, if an enqueuer halts after it has been attributed a rank~\idx\ but
+%before it updates the corresponding slot, then no dequeuer will be able to
+%dequeue the corresponding item, hence the tail will remain blocked at rank~\idx\
+%and the head will remain blocked at $\idx+\capacity$ (the queue will remain
+%full forever).
+%
+%Symmetrically, if a dequeuer halts after it has been attributed a rank~\idx\ but
+%before it updates the corresponding slot, then no enqueuer will be able to reuse
+%the slot for an item of rank~$\idx+\capacity$, hence the head will remain
+%blocked at rank~$\idx+\capacity$ and the tail will remain blocked at
+%$\idx+\capacity$ (the queue will remain empty forever).