Merge tag 'lkmm.2024.05.10a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull LKMM documentation updates from Paul McKenney:
 "This upgrades LKMM documentation, perhaps most notably adding a number
  of litmus tests illustrating cmpxchg() ordering properties.

  TL;DR: Failing cmpxchg() operations provide no ordering"

* tag 'lkmm.2024.05.10a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  Documentation/litmus-tests: Make cmpxchg() tests safe for klitmus
  Documentation/atomic_t: Emphasize that failed atomic operations give no ordering
  Documentation/litmus-tests: Demonstrate unordered failing cmpxchg
  Documentation/litmus-tests: Add locking tests to README

Author: torvalds

Documentation/atomic_t.txt

@@ -171,14 +171,14 @@ The rule of thumb:
  - RMW operations that are conditional are unordered on FAILURE,
    otherwise the above rules apply.
 
-Except of course when an operation has an explicit ordering like:
+Except of course when a successful operation has an explicit ordering like:
 
  {}_relaxed: unordered
  {}_acquire: the R of the RMW (or atomic_read) is an ACQUIRE
  {}_release: the W of the RMW (or atomic_set)  is a  RELEASE
 
 Where 'unordered' is against other memory locations. Address dependencies are
-not defeated.
+not defeated.  Conditional operations are still unordered on FAILURE.
 
 Fully ordered primitives are ordered against everything prior and everything
 subsequent. Therefore a fully ordered primitive is like having an smp_mb()

Documentation/litmus-tests/README

@@ -21,6 +21,51 @@ Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus
     Test that atomic_set() cannot break the atomicity of atomic RMWs.
     NOTE: Require herd7 7.56 or later which supports "(void)expr".
 
+cmpxchg-fail-ordered-1.litmus
+    Demonstrate that a failing cmpxchg() operation acts as a full barrier
+    when followed by smp_mb__after_atomic().
+
+cmpxchg-fail-ordered-2.litmus
+    Demonstrate that a failing cmpxchg() operation acts as an acquire
+    operation when followed by smp_mb__after_atomic().
+
+cmpxchg-fail-unordered-1.litmus
+    Demonstrate that a failing cmpxchg() operation does not act as a
+    full barrier.
+
+cmpxchg-fail-unordered-2.litmus
+    Demonstrate that a failing cmpxchg() operation does not act as an
+    acquire operation.
+
+
+locking (/locking directory)
+----------------------------
+
+DCL-broken.litmus
+    Demonstrates that double-checked locking needs more than just
+    the obvious lock acquisitions and releases.
+
+DCL-fixed.litmus
+    Demonstrates corrected double-checked locking that uses
+    smp_store_release() and smp_load_acquire() in addition to the
+    obvious lock acquisitions and releases.
+
+RM-broken.litmus
+    Demonstrates problems with "roach motel" locking, where code is
+    freely moved into lock-based critical sections.  This example also
+    shows how to use the "filter" clause to discard executions that
+    would be excluded by other code not modeled in the litmus test.
+    Note also that this "roach motel" optimization is emulated by
+    physically moving P1()'s two reads from x under the lock.
+
+    What is a roach motel?  This is from an old advertisement for
+    a cockroach trap, much later featured in one of the "Men in
+    Black" movies.  "The roaches check in.  They don't check out."
+
+RM-fixed.litmus
+    The counterpart to RM-broken.litmus, showing P0()'s two loads from
+    x safely outside of the critical section.
+
 
 RCU (/rcu directory)
 --------------------

Documentation/litmus-tests/atomic/cmpxchg-fail-ordered-1.litmus

@@ -0,0 +1,35 @@
+C cmpxchg-fail-ordered-1
+
+(*
+ * Result: Never
+ *
+ * Demonstrate that a failing cmpxchg() operation will act as a full
+ * barrier when followed by smp_mb__after_atomic().
+ *)
+
+{}
+
+P0(int *x, int *y, int *z)
+{
+	int r0;
+	int r1;
+
+	WRITE_ONCE(*x, 1);
+	r1 = cmpxchg(z, 1, 0);
+	smp_mb__after_atomic();
+	r0 = READ_ONCE(*y);
+}
+
+P1(int *x, int *y, int *z)
+{
+	int r0;
+	int r1;
+
+	WRITE_ONCE(*y, 1);
+	r1 = cmpxchg(z, 1, 0);
+	smp_mb__after_atomic();
+	r0 = READ_ONCE(*x);
+}
+
+locations[0:r1;1:r1]
+exists (0:r0=0 /\ 1:r0=0)

Documentation/litmus-tests/atomic/cmpxchg-fail-ordered-2.litmus

@@ -0,0 +1,30 @@
+C cmpxchg-fail-ordered-2
+
+(*
+ * Result: Never
+ *
+ * Demonstrate use of smp_mb__after_atomic() to make a failing cmpxchg
+ * operation have acquire ordering.
+ *)
+
+{}
+
+P0(int *x, int *y)
+{
+	int r1;
+
+	WRITE_ONCE(*x, 1);
+	r1 = cmpxchg(y, 0, 1);
+}
+
+P1(int *x, int *y)
+{
+	int r1;
+	int r2;
+
+	r1 = cmpxchg(y, 0, 1);
+	smp_mb__after_atomic();
+	r2 = READ_ONCE(*x);
+}
+
+exists (0:r1=0 /\ 1:r1=1 /\ 1:r2=0)

Documentation/litmus-tests/atomic/cmpxchg-fail-unordered-1.litmus

@@ -0,0 +1,34 @@
+C cmpxchg-fail-unordered-1
+
+(*
+ * Result: Sometimes
+ *
+ * Demonstrate that a failing cmpxchg() operation does not act as a
+ * full barrier.  (In contrast, a successful cmpxchg() does act as a
+ * full barrier.)
+ *)
+
+{}
+
+P0(int *x, int *y, int *z)
+{
+	int r0;
+	int r1;
+
+	WRITE_ONCE(*x, 1);
+	r1 = cmpxchg(z, 1, 0);
+	r0 = READ_ONCE(*y);
+}
+
+P1(int *x, int *y, int *z)
+{
+	int r0;
+	int r1;
+
+	WRITE_ONCE(*y, 1);
+	r1 = cmpxchg(z, 1, 0);
+	r0 = READ_ONCE(*x);
+}
+
+locations[0:r1;1:r1]
+exists (0:r0=0 /\ 1:r0=0)

Documentation/litmus-tests/atomic/cmpxchg-fail-unordered-2.litmus

@@ -0,0 +1,30 @@
+C cmpxchg-fail-unordered-2
+
+(*
+ * Result: Sometimes
+ *
+ * Demonstrate that a failing cmpxchg() operation does not act as either
+ * an acquire release operation.  (In contrast, a successful cmpxchg()
+ * does act as both an acquire and a release operation.)
+ *)
+
+{}
+
+P0(int *x, int *y)
+{
+	int r1;
+
+	WRITE_ONCE(*x, 1);
+	r1 = cmpxchg(y, 0, 1);
+}
+
+P1(int *x, int *y)
+{
+	int r1;
+	int r2;
+
+	r1 = cmpxchg(y, 0, 1);
+	r2 = READ_ONCE(*x);
+}
+
+exists (0:r1=0 /\ 1:r1=1 /\ 1:r2=0)