[PATCH memory-model 0/3] LKMM updates for v5.17

[PATCH memory-model 0/3] LKMM updates for v5.17

[Paul E. McKenney]

Hello!

This series provides additional documentation and examples for the
Linux-kernel memory model (LKMM):

1.	Provide extra ordering for unlock+lock pair on the same CPU,
	courtesy of Boqun Feng.

2.	Describe the requirement of the litmus-tests directory, courtesy
	of Boqun Feng.

3.	Add two tests for unlock(A)+lock(B) ordering, courtesy of
	Boqun Feng.

						Thanx, Paul

------------------------------------------------------------------------

 Documentation/explanation.txt                              |   44 +++++++------
 README                                                     |   12 +++
 linux-kernel.cat                                           |    6 -
 litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus    |   35 ++++++++++
 litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus |   33 +++++++++
 litmus-tests/README                                        |    8 ++
 6 files changed, 116 insertions(+), 22 deletions(-)

[PATCH memory-model 1/3] tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU

[Paul E. McKenney]

From: Boqun Feng <boqun.feng@gmail.com>

A recent discussion[1] shows that we are in favor of strengthening the
ordering of unlock + lock on the same CPU: a unlock and a po-after lock
should provide the so-called RCtso ordering, that is a memory access S
po-before the unlock should be ordered against a memory access R
po-after the lock, unless S is a store and R is a load.

The strengthening meets programmers' expection that "sequence of two
locked regions to be ordered wrt each other" (from Linus), and can
reduce the mental burden when using locks. Therefore add it in LKMM.

[1]: https://lore.kernel.org/lkml/20210909185937.GA12379@rowland.harvard.edu/

Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com> (RISC-V)
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 .../Documentation/explanation.txt             | 44 +++++++++++--------
 tools/memory-model/linux-kernel.cat           |  6 +--
 2 files changed, 28 insertions(+), 22 deletions(-)

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index 5d72f3112e565..394ee57d58f2f 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/tools/memory-model/Documentation/explanation.txt
@@ -1813,15 +1813,16 @@ spin_trylock() -- we can call these things lock-releases and
 lock-acquires -- have two properties beyond those of ordinary releases
 and acquires.
 
-First, when a lock-acquire reads from a lock-release, the LKMM
-requires that every instruction po-before the lock-release must
-execute before any instruction po-after the lock-acquire.  This would
-naturally hold if the release and acquire operations were on different
-CPUs, but the LKMM says it holds even when they are on the same CPU.
-For example:
+First, when a lock-acquire reads from or is po-after a lock-release,
+the LKMM requires that every instruction po-before the lock-release
+must execute before any instruction po-after the lock-acquire.  This
+would naturally hold if the release and acquire operations were on
+different CPUs and accessed the same lock variable, but the LKMM says
+it also holds when they are on the same CPU, even if they access
+different lock variables.  For example:
 
 	int x, y;
-	spinlock_t s;
+	spinlock_t s, t;
 
 	P0()
 	{
@@ -1830,9 +1831,9 @@ For example:
 		spin_lock(&s);
 		r1 = READ_ONCE(x);
 		spin_unlock(&s);
-		spin_lock(&s);
+		spin_lock(&t);
 		r2 = READ_ONCE(y);
-		spin_unlock(&s);
+		spin_unlock(&t);
 	}
 
 	P1()
@@ -1842,10 +1843,10 @@ For example:
 		WRITE_ONCE(x, 1);
 	}
 
-Here the second spin_lock() reads from the first spin_unlock(), and
-therefore the load of x must execute before the load of y.  Thus we
-cannot have r1 = 1 and r2 = 0 at the end (this is an instance of the
-MP pattern).
+Here the second spin_lock() is po-after the first spin_unlock(), and
+therefore the load of x must execute before the load of y, even though
+the two locking operations use different locks.  Thus we cannot have
+r1 = 1 and r2 = 0 at the end (this is an instance of the MP pattern).
 
 This requirement does not apply to ordinary release and acquire
 fences, only to lock-related operations.  For instance, suppose P0()
@@ -1872,13 +1873,13 @@ instructions in the following order:
 
 and thus it could load y before x, obtaining r2 = 0 and r1 = 1.
 
-Second, when a lock-acquire reads from a lock-release, and some other
-stores W and W' occur po-before the lock-release and po-after the
-lock-acquire respectively, the LKMM requires that W must propagate to
-each CPU before W' does.  For example, consider:
+Second, when a lock-acquire reads from or is po-after a lock-release,
+and some other stores W and W' occur po-before the lock-release and
+po-after the lock-acquire respectively, the LKMM requires that W must
+propagate to each CPU before W' does.  For example, consider:
 
 	int x, y;
-	spinlock_t x;
+	spinlock_t s;
 
 	P0()
 	{
@@ -1908,7 +1909,12 @@ each CPU before W' does.  For example, consider:
 
 If r1 = 1 at the end then the spin_lock() in P1 must have read from
 the spin_unlock() in P0.  Hence the store to x must propagate to P2
-before the store to y does, so we cannot have r2 = 1 and r3 = 0.
+before the store to y does, so we cannot have r2 = 1 and r3 = 0.  But
+if P1 had used a lock variable different from s, the writes could have
+propagated in either order.  (On the other hand, if the code in P0 and
+P1 had all executed on a single CPU, as in the example before this
+one, then the writes would have propagated in order even if the two
+critical sections used different lock variables.)
 
 These two special requirements for lock-release and lock-acquire do
 not arise from the operational model.  Nevertheless, kernel developers
diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index 2a9b4fe4a84eb..d70315fddef6e 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -27,7 +27,7 @@ include "lock.cat"
 (* Release Acquire *)
 let acq-po = [Acquire] ; po ; [M]
 let po-rel = [M] ; po ; [Release]
-let po-unlock-rf-lock-po = po ; [UL] ; rf ; [LKR] ; po
+let po-unlock-lock-po = po ; [UL] ; (po|rf) ; [LKR] ; po
 
 (* Fences *)
 let R4rmb = R \ Noreturn	(* Reads for which rmb works *)
@@ -70,12 +70,12 @@ let rwdep = (dep | ctrl) ; [W]
 let overwrite = co | fr
 let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
 let to-r = addr | (dep ; [Marked] ; rfi)
-let ppo = to-r | to-w | fence | (po-unlock-rf-lock-po & int)
+let ppo = to-r | to-w | fence | (po-unlock-lock-po & int)
 
 (* Propagation: Ordering from release operations and strong fences. *)
 let A-cumul(r) = (rfe ; [Marked])? ; r
 let cumul-fence = [Marked] ; (A-cumul(strong-fence | po-rel) | wmb |
-	po-unlock-rf-lock-po) ; [Marked]
+	po-unlock-lock-po) ; [Marked]
 let prop = [Marked] ; (overwrite & ext)? ; cumul-fence* ;
 	[Marked] ; rfe? ; [Marked]
 
-- 
2.31.1.189.g2e36527f23

[PATCH memory-model 2/3] tools/memory-model: doc: Describe the requirement of the litmus-tests directory

[Paul E. McKenney]

From: Boqun Feng <boqun.feng@gmail.com>

It's better that we have some "standard" about which test should be put
in the litmus-tests directory because it helps future contributors
understand whether they should work on litmus-tests in kernel or Paul's
GitHub repo. Therefore explain a little bit on what a "representative"
litmus test is.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 tools/memory-model/README | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/memory-model/README b/tools/memory-model/README
index 9a84c45504ab6..9edd402704c4f 100644
--- a/tools/memory-model/README
+++ b/tools/memory-model/README
@@ -195,6 +195,18 @@ litmus-tests
 	are listed in litmus-tests/README.  A great deal more litmus
 	tests are available at https://github.com/paulmckrcu/litmus.
 
+	By "representative", it means the one in the litmus-tests
+	directory is:
+
+		1) simple, the number of threads should be relatively
+		   small and each thread function should be relatively
+		   simple.
+		2) orthogonal, there should be no two litmus tests
+		   describing the same aspect of the memory model.
+		3) textbook, developers can easily copy-paste-modify
+		   the litmus tests to use the patterns on their own
+		   code.
+
 lock.cat
 	Provides a front-end analysis of lock acquisition and release,
 	for example, associating a lock acquisition with the preceding
-- 
2.31.1.189.g2e36527f23

[PATCH memory-model 3/3] tools/memory-model: litmus: Add two tests for unlock(A)+lock(B) ordering

[Paul E. McKenney]

From: Boqun Feng <boqun.feng@gmail.com>

The memory model has been updated to provide a stronger ordering
guarantee for unlock(A)+lock(B) on the same CPU/thread. Therefore add
two litmus tests describing this new guarantee, these tests are simple
yet can clearly show the usage of the new guarantee, also they can serve
as the self tests for the modification in the model.

Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 ...LB+unlocklockonceonce+poacquireonce.litmus | 35 +++++++++++++++++++
 ...unlocklockonceonce+fencermbonceonce.litmus | 33 +++++++++++++++++
 tools/memory-model/litmus-tests/README        |  8 +++++
 3 files changed, 76 insertions(+)
 create mode 100644 tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
 create mode 100644 tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus

diff --git a/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
new file mode 100644
index 0000000000000..eb34123a6ffe0
--- /dev/null
+++ b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
@@ -0,0 +1,35 @@
+C LB+unlocklockonceonce+poacquireonce
+
+(*
+ * Result: Never
+ *
+ * If two locked critical sections execute on the same CPU, all accesses
+ * in the first must execute before any accesses in the second, even if the
+ * critical sections are protected by different locks.  Note: Even when a
+ * write executes before a read, their memory effects can be reordered from
+ * the viewpoint of another CPU (the kind of reordering allowed by TSO).
+ *)
+
+{}
+
+P0(spinlock_t *s, spinlock_t *t, int *x, int *y)
+{
+	int r1;
+
+	spin_lock(s);
+	r1 = READ_ONCE(*x);
+	spin_unlock(s);
+	spin_lock(t);
+	WRITE_ONCE(*y, 1);
+	spin_unlock(t);
+}
+
+P1(int *x, int *y)
+{
+	int r2;
+
+	r2 = smp_load_acquire(y);
+	WRITE_ONCE(*x, 1);
+}
+
+exists (0:r1=1 /\ 1:r2=1)
diff --git a/tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus b/tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus
new file mode 100644
index 0000000000000..2feb1398be716
--- /dev/null
+++ b/tools/memory-model/litmus-tests/MP+unlocklockonceonce+fencermbonceonce.litmus
@@ -0,0 +1,33 @@
+C MP+unlocklockonceonce+fencermbonceonce
+
+(*
+ * Result: Never
+ *
+ * If two locked critical sections execute on the same CPU, stores in the
+ * first must propagate to each CPU before stores in the second do, even if
+ * the critical sections are protected by different locks.
+ *)
+
+{}
+
+P0(spinlock_t *s, spinlock_t *t, int *x, int *y)
+{
+	spin_lock(s);
+	WRITE_ONCE(*x, 1);
+	spin_unlock(s);
+	spin_lock(t);
+	WRITE_ONCE(*y, 1);
+	spin_unlock(t);
+}
+
+P1(int *x, int *y)
+{
+	int r1;
+	int r2;
+
+	r1 = READ_ONCE(*y);
+	smp_rmb();
+	r2 = READ_ONCE(*x);
+}
+
+exists (1:r1=1 /\ 1:r2=0)
diff --git a/tools/memory-model/litmus-tests/README b/tools/memory-model/litmus-tests/README
index 681f9067fa9ed..d311a0ff1ae64 100644
--- a/tools/memory-model/litmus-tests/README
+++ b/tools/memory-model/litmus-tests/README
@@ -63,6 +63,10 @@ LB+poonceonces.litmus
 	As above, but with store-release replaced with WRITE_ONCE()
 	and load-acquire replaced with READ_ONCE().
 
+LB+unlocklockonceonce+poacquireonce.litmus
+	Does a unlock+lock pair provides ordering guarantee between a
+	load and a store?
+
 MP+onceassign+derefonce.litmus
 	As below, but with rcu_assign_pointer() and an rcu_dereference().
 
@@ -90,6 +94,10 @@ MP+porevlocks.litmus
 	As below, but with the first access of the writer process
 	and the second access of reader process protected by a lock.
 
+MP+unlocklockonceonce+fencermbonceonce.litmus
+	Does a unlock+lock pair provides ordering guarantee between a
+	store and another store?
+
 MP+fencewmbonceonce+fencermbonceonce.litmus
 	Does a smp_wmb() (between the stores) and an smp_rmb() (between
 	the loads) suffice for the message-passing litmus test, where one
-- 
2.31.1.189.g2e36527f23