EXP litmus_tests: Add comments explaining tests' purposes

This commit adds comments to the litmus tests summarizing what these
tests are intended to demonstrate.

[ paulmck: Apply Andrea's and Alan's feedback. ]
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: akiyks@gmail.com
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: nborisov@suse.com
Cc: npiggin@gmail.com
Cc: parri.andrea@gmail.com
Cc: stern@rowland.harvard.edu
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1519169112-20593-4-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/memory-model/litmus-tests/CoRR+poonceonce+Once.litmus

@@ -1,5 +1,12 @@
 C CoRR+poonceonce+Once
 
+(*
+ * Result: Never
+ *
+ * Test of read-read coherence, that is, whether or not two successive
+ * reads from the same variable are ordered.
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/CoRW+poonceonce+Once.litmus

@@ -1,5 +1,12 @@
 C CoRW+poonceonce+Once
 
+(*
+ * Result: Never
+ *
+ * Test of read-write coherence, that is, whether or not a read from
+ * a given variable and a later write to that same variable are ordered.
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/CoWR+poonceonce+Once.litmus

@@ -1,5 +1,12 @@
 C CoWR+poonceonce+Once
 
+(*
+ * Result: Never
+ *
+ * Test of write-read coherence, that is, whether or not a write to a
+ * given variable and a later read from that same variable are ordered.
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/CoWW+poonceonce.litmus

@@ -1,5 +1,12 @@
 C CoWW+poonceonce
 
+(*
+ * Result: Never
+ *
+ * Test of write-write coherence, that is, whether or not two successive
+ * writes to the same variable are ordered.
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/IRIW+mbonceonces+OnceOnce.litmus

@@ -1,5 +1,15 @@
 C IRIW+mbonceonces+OnceOnce
 
+(*
+ * Result: Never
+ *
+ * Test of independent reads from independent writes with smp_mb()
+ * between each pairs of reads.  In other words, is smp_mb() sufficient to
+ * cause two different reading processes to agree on the order of a pair
+ * of writes, where each write is to a different variable by a different
+ * process?
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/IRIW+poonceonces+OnceOnce.litmus

@@ -1,5 +1,15 @@
 C IRIW+poonceonces+OnceOnce
 
+(*
+ * Result: Sometimes
+ *
+ * Test of independent reads from independent writes with nothing
+ * between each pairs of reads.  In other words, is anything at all
+ * needed to cause two different reading processes to agree on the order
+ * of a pair of writes, where each write is to a different variable by a
+ * different process?
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/ISA2+poonceonces.litmus

@@ -1,5 +1,14 @@
 C ISA2+poonceonces
 
+(*
+ * Result: Sometimes
+ *
+ * Given a release-acquire chain ordering the first process's store
+ * against the last process's load, is ordering preserved if all of the
+ * smp_store_release() invocations are replaced by WRITE_ONCE() and all
+ * of the smp_load_acquire() invocations are replaced by READ_ONCE()?
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus

@@ -1,5 +1,16 @@
 C ISA2+pooncerelease+poacquirerelease+poacquireonce
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that a release-acquire chain suffices
+ * to order P0()'s initial write against P2()'s final read.  The reason
+ * that the release-acquire chain suffices is because in all but one
+ * case (P2() to P0()), each process reads from the preceding process's
+ * write.  In memory-model-speak, there is only one non-reads-from
+ * (AKA non-rf) link, so release-acquire is all that is needed.
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/LB+ctrlonceonce+mbonceonce.litmus

@@ -1,5 +1,16 @@
 C LB+ctrlonceonce+mbonceonce
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that lightweight ordering suffices for
+ * the load-buffering pattern, in other words, preventing all processes
+ * reading from the preceding process's write.  In this example, the
+ * combination of a control dependency and a full memory barrier are enough
+ * to do the trick.  (But the full memory barrier could be replaced with
+ * another control dependency and order would still be maintained.)
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/LB+poacquireonce+pooncerelease.litmus

@@ -1,5 +1,13 @@
 C LB+poacquireonce+pooncerelease
 
+(*
+ * Result: Never
+ *
+ * Does a release-acquire pair suffice for the load-buffering litmus
+ * test, where each process reads from one of two variables then writes
+ * to the other?
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/LB+poonceonces.litmus

@@ -1,5 +1,12 @@
 C LB+poonceonces
 
+(*
+ * Result: Sometimes
+ *
+ * Can the counter-intuitive outcome for the load-buffering pattern
+ * be prevented even with no explicit ordering?
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/MP+onceassign+derefonce.litmus

@@ -1,4 +1,13 @@
-C MP+onceassign+derefonce.litmus
+C MP+onceassign+derefonce
+
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that rcu_assign_pointer() and
+ * rcu_dereference() suffice to ensure that an RCU reader will not see
+ * pre-initialization garbage when it traverses an RCU-protected data
+ * structure containing a newly inserted element.
+ *)
 
 {
 y=z;

tools/memory-model/litmus-tests/MP+polocks.litmus

@@ -1,5 +1,16 @@
 C MP+polocks
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates how lock acquisitions and releases can
+ * stand in for smp_load_acquire() and smp_store_release(), respectively.
+ * In other words, when holding a given lock (or indeed after releasing a
+ * given lock), a CPU is not only guaranteed to see the accesses that other
+ * CPUs made while previously holding that lock, it is also guaranteed
+ * to see all prior accesses by those other CPUs.
+ *)
+
 {}
 
 P0(int *x, int *y, spinlock_t *mylock)

tools/memory-model/litmus-tests/MP+poonceonces.litmus

@@ -1,5 +1,12 @@
 C MP+poonceonces
 
+(*
+ * Result: Maybe
+ *
+ * Can the counter-intuitive message-passing outcome be prevented with
+ * no ordering at all?
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/MP+pooncerelease+poacquireonce.litmus

@@ -1,5 +1,13 @@
 C MP+pooncerelease+poacquireonce
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that smp_store_release() and
+ * smp_load_acquire() provide sufficient ordering for the message-passing
+ * pattern.
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/MP+porevlocks.litmus

@@ -1,5 +1,16 @@
 C MP+porevlocks
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates how lock acquisitions and releases can
+ * stand in for smp_load_acquire() and smp_store_release(), respectively.
+ * In other words, when holding a given lock (or indeed after releasing a
+ * given lock), a CPU is not only guaranteed to see the accesses that other
+ * CPUs made while previously holding that lock, it is also guaranteed to
+ * see all prior accesses by those other CPUs.
+ *)
+
 {}
 
 P0(int *x, int *y, spinlock_t *mylock)

tools/memory-model/litmus-tests/MP+wmbonceonce+rmbonceonce.litmus

@@ -1,5 +1,13 @@
 C MP+wmbonceonce+rmbonceonce
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that smp_wmb() and smp_rmb() provide
+ * sufficient ordering for the message-passing pattern.  However, it
+ * is usually better to use smp_store_release() and smp_load_acquire().
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/R+mbonceonces.litmus

@@ -1,5 +1,14 @@
 C R+mbonceonces
 
+(*
+ * Result: Never
+ *
+ * This is the fully ordered (via smp_mb()) version of one of the classic
+ * counterintuitive litmus tests that illustrates the effects of store
+ * propagation delays.  Note that weakening either of the barriers would
+ * cause the resulting test to be allowed.
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/R+poonceonces.litmus

@@ -1,5 +1,13 @@
 C R+poonceonces
 
+(*
+ * Result: Sometimes
+ *
+ * This is the unordered (thus lacking smp_mb()) version of one of the
+ * classic counterintuitive litmus tests that illustrates the effects of
+ * store propagation delays.
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/S+poonceonces.litmus

@@ -1,5 +1,14 @@
 C S+poonceonces
 
+(*
+ * Result: Sometimes
+ *
+ * Starting with a two-process release-acquire chain ordering P0()'s
+ * first store against P1()'s final load, if the smp_store_release()
+ * is replaced by WRITE_ONCE() and the smp_load_acquire() replaced by
+ * READ_ONCE(), is ordering preserved?
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/S+wmbonceonce+poacquireonce.litmus

@@ -1,5 +1,12 @@
 C S+wmbonceonce+poacquireonce
 
+(*
+ * Result: Never
+ *
+ * Can a smp_wmb(), instead of a release, and an acquire order a prior
+ * store against a subsequent store?
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/SB+mbonceonces.litmus

@@ -1,5 +1,14 @@
 C SB+mbonceonces
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates that full memory barriers suffice to
+ * order the store-buffering pattern, where each process writes to the
+ * variable that the preceding process reads.  (Locking and RCU can also
+ * suffice, but not much else.)
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/SB+poonceonces.litmus

@@ -1,5 +1,13 @@
 C SB+poonceonces
 
+(*
+ * Result: Sometimes
+ *
+ * This litmus test demonstrates that at least some ordering is required
+ * to order the store-buffering pattern, where each process writes to the
+ * variable that the preceding process reads.
+ *)
+
 {}
 
 P0(int *x, int *y)

tools/memory-model/litmus-tests/WRC+poonceonces+Once.litmus

@@ -1,5 +1,13 @@
 C WRC+poonceonces+Once
 
+(*
+ * Result: Sometimes
+ *
+ * This litmus test is an extension of the message-passing pattern,
+ * where the first write is moved to a separate process.  Note that this
+ * test has no ordering at all.
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/WRC+pooncerelease+rmbonceonce+Once.litmus

@@ -1,5 +1,13 @@
 C WRC+pooncerelease+rmbonceonce+Once
 
+(*
+ * Result: Never
+ *
+ * This litmus test is an extension of the message-passing pattern, where
+ * the first write is moved to a separate process.  Because it features
+ * a release and a read memory barrier, it should be forbidden.
+ *)
+
 {}
 
 P0(int *x)

tools/memory-model/litmus-tests/Z6.0+pooncelock+poonceLock+pombonce.litmus

@@ -1,5 +1,14 @@
 C Z6.0+pooncelock+poonceLock+pombonce
 
+(*
+ * Result: Never
+ *
+ * This litmus test demonstrates how smp_mb__after_spinlock() may be
+ * used to ensure that accesses in different critical sections for a
+ * given lock running on different CPUs are nevertheless seen in order
+ * by CPUs not holding that lock.
+ *)
+
 {}
 
 P0(int *x, int *y, spinlock_t *mylock)

tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus

@@ -1,5 +1,13 @@
 C Z6.0+pooncelock+pooncelock+pombonce
 
+(*
+ * Result: Sometimes
+ *
+ * This example demonstrates that a pair of accesses made by different
+ * processes each while holding a given lock will not necessarily be
+ * seen as ordered by a third process not holding that lock.
+ *)
+
 {}
 
 P0(int *x, int *y, spinlock_t *mylock)

tools/memory-model/litmus-tests/Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus

@@ -1,5 +1,19 @@
 C Z6.0+pooncerelease+poacquirerelease+mbonceonce
 
+(*
+ * Result: Sometimes
+ *
+ * This litmus test shows that a release-acquire chain, while sufficient
+ * when there is but one non-reads-from (AKA non-rf) link, does not suffice
+ * if there is more than one.  Of the three processes, only P1() reads from
+ * P0's write, which means that there are two non-rf links: P1() to P2()
+ * is a write-to-write link (AKA a "coherence" or just "co" link) and P2()
+ * to P0() is a read-to-write link (AKA a "from-reads" or just "fr" link).
+ * When there are two or more non-rf links, you typically will need one
+ * full barrier for each non-rf link.  (Exceptions include some cases
+ * involving locking.)
+ *)
+
 {}
 
 P0(int *x, int *y)