Merge tag 'lkmm.2022.03.13a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull memory model doc update from Paul McKenney:
 "An improved explanation of syntactic and semantic dependencies from
  Alan Stern"

* tag 'lkmm.2022.03.13a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  tools/memory-model: Explain syntactic and semantic dependencies

Author: torvalds

tools/memory-model/Documentation/explanation.txt

@@ -485,6 +485,57 @@ have R ->po X.  It wouldn't make sense for a computation to depend
 somehow on a value that doesn't get loaded from shared memory until
 later in the code!
 
+Here's a trick question: When is a dependency not a dependency?  Answer:
+When it is purely syntactic rather than semantic.  We say a dependency
+between two accesses is purely syntactic if the second access doesn't
+actually depend on the result of the first.  Here is a trivial example:
+
+	r1 = READ_ONCE(x);
+	WRITE_ONCE(y, r1 * 0);
+
+There appears to be a data dependency from the load of x to the store
+of y, since the value to be stored is computed from the value that was
+loaded.  But in fact, the value stored does not really depend on
+anything since it will always be 0.  Thus the data dependency is only
+syntactic (it appears to exist in the code) but not semantic (the
+second access will always be the same, regardless of the value of the
+first access).  Given code like this, a compiler could simply discard
+the value returned by the load from x, which would certainly destroy
+any dependency.  (The compiler is not permitted to eliminate entirely
+the load generated for a READ_ONCE() -- that's one of the nice
+properties of READ_ONCE() -- but it is allowed to ignore the load's
+value.)
+
+It's natural to object that no one in their right mind would write
+code like the above.  However, macro expansions can easily give rise
+to this sort of thing, in ways that often are not apparent to the
+programmer.
+
+Another mechanism that can lead to purely syntactic dependencies is
+related to the notion of "undefined behavior".  Certain program
+behaviors are called "undefined" in the C language specification,
+which means that when they occur there are no guarantees at all about
+the outcome.  Consider the following example:
+
+	int a[1];
+	int i;
+
+	r1 = READ_ONCE(i);
+	r2 = READ_ONCE(a[r1]);
+
+Access beyond the end or before the beginning of an array is one kind
+of undefined behavior.  Therefore the compiler doesn't have to worry
+about what will happen if r1 is nonzero, and it can assume that r1
+will always be zero regardless of the value actually loaded from i.
+(If the assumption turns out to be wrong the resulting behavior will
+be undefined anyway, so the compiler doesn't care!)  Thus the value
+from the load can be discarded, breaking the address dependency.
+
+The LKMM is unaware that purely syntactic dependencies are different
+from semantic dependencies and therefore mistakenly predicts that the
+accesses in the two examples above will be ordered.  This is another
+example of how the compiler can undermine the memory model.  Be warned.
+
 
 THE READS-FROM RELATION: rf, rfi, and rfe
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