explain example a little more

lectures/concurrency/index.html

@@ -556,7 +556,8 @@ <h2>When is synchronization needed?</h2>
 do not propagate immediately to main memory or to the caches of other processors.
 For example, consider
 two threads executing the following code in parallel. Say the values of <code>x</code> and <code>y</code>
-are initially 0.
+are initially 0. The idea is for Thread 2 to wait until Thread 1 finishes some work. It
+signals that it is done by setting the variable <code>x</code> to 1:
 </p>
 <table class=sidebysidecode><tr><td>
 Thread 1:
@@ -576,11 +577,12 @@ <h2>When is synchronization needed?</h2>
 Thread 2 could observe the update to <code>x</code> <strong>before</strong> the update to <code>y</code>.
 The fact that Thread 1 assigned to <code>y</code> before
 it assigned 1 to <code>x</code> does not matter! This behavior can and does happen frequently
-with modern hardware. When it happens is hardware-dependent, so that code that seems to work
-correctly on one processor (say, an Intel x86 processor) may fail to work on a machine using
-an ARM processor. This bizarre state of affairs shows that programming with
-concurrency can be highly counterintuitive, and that one must never rely on naive assumptions
-about the order of events.
+with modern hardware. When it happens is hardware-dependent, so that code that
+seems to work correctly on one processor (say, an Intel x86 processor) may fail
+to work on a machine using an ARM processor, which makes weaker guarantees.
+This bizarre state of affairs shows that programming with concurrency can be
+highly counterintuitive, and that it is easy to make bad assumptions about the
+order of events.
 </p>
 <p>
 The most reliable way

lectures/synchronization/index.html

@@ -124,6 +124,7 @@ <h2>Condition variables</h2>
 <p>
 Given that synchronization is needed for threads to see each other's effects, is there
 a way to successfully implement the earlier example that broke due to weak memory consistency?
+Recall that the idea was to have one thread wait until another one finished some updates:
 </p>
 
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