Re: [GIT PULL tools] Linux kernel memory model

[Andrea Parri <parri.andrea@gmail.com>]

On Sun, Feb 04, 2018 at 02:17:00AM -0800, Paul E. McKenney wrote:

[...]

> And here is the updated commit adding comments to the litmus test,
> which adds comments for the three litmus tests added above.  I have also
> marked this commit with "EXP" indicating that it has not yet had time
> for review.  This marking appears only on my commits -- others' commits
> don't get pulled until there has been time for review.  (I have to put
> my commits somewhere, and maintaining two different branches would be
> a real mess given the likelihood of depeendencies among comits.)
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 49af6e403afab890a54518980d345431d74234a4
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date:   Sat Feb 3 00:04:49 2018 -0800
> 
>     EXP litmus_tests:  Add comments explaining tests' purposes
>     
>     This commit adds comments to the litmus tests summarizing what these
>     tests are intended to demonstrate.
>     
>     Suggested-by: Ingo Molnar <mingo@kernel.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> diff --git a/tools/memory-model/litmus-tests/CoRR+poonceonce+Once.litmus b/tools/memory-model/litmus-tests/CoRR+poonceonce+Once.litmus
> index 5b83d57f6ac5..8e8ae8989085 100644
> --- a/tools/memory-model/litmus-tests/CoRR+poonceonce+Once.litmus
> +++ b/tools/memory-model/litmus-tests/CoRR+poonceonce+Once.litmus
> @@ -1,5 +1,11 @@
>  C CoRR+poonceonce+Once
>  
> +(*
> + * Test of read-read coherence, that is, whether or not two successive
> + * reads from the same variable are ordered.  They should be ordered,
> + * that is, this test should be forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/CoRW+poonceonce+Once.litmus b/tools/memory-model/litmus-tests/CoRW+poonceonce+Once.litmus
> index fab91c13d52c..0078ecd76f5e 100644
> --- a/tools/memory-model/litmus-tests/CoRW+poonceonce+Once.litmus
> +++ b/tools/memory-model/litmus-tests/CoRW+poonceonce+Once.litmus
> @@ -1,5 +1,11 @@
>  C CoRW+poonceonce+Once
>  
> +(*
> + * Test of read-write coherence, that is, whether or not a read from a
> + * given variable followed by a write to that same variable are ordered.
> + * This should be ordered, that is, this test should be forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/CoWR+poonceonce+Once.litmus b/tools/memory-model/litmus-tests/CoWR+poonceonce+Once.litmus
> index 6a35ec2042ea..c9d342c8fbec 100644
> --- a/tools/memory-model/litmus-tests/CoWR+poonceonce+Once.litmus
> +++ b/tools/memory-model/litmus-tests/CoWR+poonceonce+Once.litmus
> @@ -1,5 +1,11 @@
>  C CoWR+poonceonce+Once
>  
> +(*
> + * Test of write-read coherence, that is, whether or not a write to a
> + * given variable followed by a read from that same variable are ordered.
> + * They should be ordered, that is, this test should be forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/CoWW+poonceonce.litmus b/tools/memory-model/litmus-tests/CoWW+poonceonce.litmus
> index 32a96b832021..ad51c7b17f7b 100644
> --- a/tools/memory-model/litmus-tests/CoWW+poonceonce.litmus
> +++ b/tools/memory-model/litmus-tests/CoWW+poonceonce.litmus
> @@ -1,5 +1,11 @@
>  C CoWW+poonceonce
>  
> +(*
> + * Test of write-write coherence, that is, whether or not two successive
> + * writes to the same variable are ordered.  They should be ordered, that
> + * is, this test should be forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/IRIW+mbonceonces+OnceOnce.litmus b/tools/memory-model/litmus-tests/IRIW+mbonceonces+OnceOnce.litmus
> index 7eba2c68992b..8a58abce69fe 100644
> --- a/tools/memory-model/litmus-tests/IRIW+mbonceonces+OnceOnce.litmus
> +++ b/tools/memory-model/litmus-tests/IRIW+mbonceonces+OnceOnce.litmus
> @@ -1,5 +1,14 @@
>  C IRIW+mbonceonces+OnceOnce
>  
> +(*
> + * 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?  The smp_mb()s should be sufficient, that is, this test should
> + * be forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/IRIW+poonceonces+OnceOnce.litmus b/tools/memory-model/litmus-tests/IRIW+poonceonces+OnceOnce.litmus
> index b0556c6c75d4..c736cd372207 100644
> --- a/tools/memory-model/litmus-tests/IRIW+poonceonces+OnceOnce.litmus
> +++ b/tools/memory-model/litmus-tests/IRIW+poonceonces+OnceOnce.litmus
> @@ -1,5 +1,14 @@
>  C IRIW+poonceonces+OnceOnce
>  
> +(*
> + * 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?  Something is needed, in other words, this test
> + * should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/ISA2+poonceonces.litmus b/tools/memory-model/litmus-tests/ISA2+poonceonces.litmus
> index 9a1a233d70c3..1f1c4220c92d 100644
> --- a/tools/memory-model/litmus-tests/ISA2+poonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/ISA2+poonceonces.litmus
> @@ -1,5 +1,13 @@
>  C ISA2+poonceonces
>  
> +(*
> + * 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 be replaced by WRITE_ONCE() and all
> + * of the smp_load_acquire() invocations be replaced by READ_ONCE()?
> + * The answer is "no", that is, this test should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus b/tools/memory-model/litmus-tests/ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus
> index 235195e87d4e..aa4b25838519 100644
> --- a/tools/memory-model/litmus-tests/ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus
> +++ b/tools/memory-model/litmus-tests/ISA2+pooncerelease+poacquirerelease+poacquireonce.litmus
> @@ -1,5 +1,14 @@
>  C ISA2+pooncerelease+poacquirerelease+poacquireonce
>  
> +(*
> + * 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)
> diff --git a/tools/memory-model/litmus-tests/LB+ctrlonceonce+mbonceonce.litmus b/tools/memory-model/litmus-tests/LB+ctrlonceonce+mbonceonce.litmus
> index dd5ac3a8974a..0b65048ad4db 100644
> --- a/tools/memory-model/litmus-tests/LB+ctrlonceonce+mbonceonce.litmus
> +++ b/tools/memory-model/litmus-tests/LB+ctrlonceonce+mbonceonce.litmus
> @@ -1,5 +1,14 @@
>  C LB+ctrlonceonce+mbonceonce
>  
> +(*
> + * 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 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)
> diff --git a/tools/memory-model/litmus-tests/LB+poacquireonce+pooncerelease.litmus b/tools/memory-model/litmus-tests/LB+poacquireonce+pooncerelease.litmus
> index 47bd61319d93..1d1f45ff1940 100644
> --- a/tools/memory-model/litmus-tests/LB+poacquireonce+pooncerelease.litmus
> +++ b/tools/memory-model/litmus-tests/LB+poacquireonce+pooncerelease.litmus
> @@ -1,5 +1,12 @@
>  C LB+poacquireonce+pooncerelease
>  
> +(*
> + * 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?  The answer is "yes", that is, this test should be
> + * forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/LB+poonceonces.litmus b/tools/memory-model/litmus-tests/LB+poonceonces.litmus
> index a5cdf027e34b..383e3e0adb4e 100644
> --- a/tools/memory-model/litmus-tests/LB+poonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/LB+poonceonces.litmus
> @@ -1,5 +1,11 @@
>  C LB+poonceonces
>  
> +(*
> + * Can the counter-intuitive outcome for the load-buffering pattern
> + * be prevented even with no explicit ordering?  The answer should be
> + * "no", that is, this test should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/MP+onceassign+derefonce.litmus b/tools/memory-model/litmus-tests/MP+onceassign+derefonce.litmus
> index 1a2fe5830381..86ddc88a26a2 100644
> --- a/tools/memory-model/litmus-tests/MP+onceassign+derefonce.litmus
> +++ b/tools/memory-model/litmus-tests/MP+onceassign+derefonce.litmus
> @@ -1,5 +1,12 @@
>  C MP+onceassign+derefonce.litmus
>  
> +(*
> + * 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;
>  z=0;
> diff --git a/tools/memory-model/litmus-tests/MP+polocks.litmus b/tools/memory-model/litmus-tests/MP+polocks.litmus
> index 5fe6f1e3c452..3e5d3fe01054 100644
> --- a/tools/memory-model/litmus-tests/MP+polocks.litmus
> +++ b/tools/memory-model/litmus-tests/MP+polocks.litmus
> @@ -1,5 +1,14 @@
>  C MP+polocks
>  
> +(*
> + * 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 relaasing a

s/relaasing/releasing


> + * given lock), a CPU is not only guaranteed to see the accesses that other
> + * CPOs made while previously holding that lock, it are also guaranteed

s/CPOs/CPUs

(same two typos for MP+porevlocks)


> + * to see all prior accesses by those other CPUs.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y, spinlock_t *mylock)
> diff --git a/tools/memory-model/litmus-tests/MP+poonceonces.litmus b/tools/memory-model/litmus-tests/MP+poonceonces.litmus
> index 46e1ac7ba126..16a1d45e3fde 100644
> --- a/tools/memory-model/litmus-tests/MP+poonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/MP+poonceonces.litmus
> @@ -1,5 +1,11 @@
>  C MP+poonceonces
>  
> +(*
> + * Can the counter-intuitive message-passing outcome be prevented with
> + * no ordering at all?  The answer should be "no", that is, this test
> + * should be prohibited.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/MP+pooncerelease+poacquireonce.litmus b/tools/memory-model/litmus-tests/MP+pooncerelease+poacquireonce.litmus
> index 0b00cc7293ba..f7fbe2636287 100644
> --- a/tools/memory-model/litmus-tests/MP+pooncerelease+poacquireonce.litmus
> +++ b/tools/memory-model/litmus-tests/MP+pooncerelease+poacquireonce.litmus
> @@ -1,5 +1,11 @@
>  C MP+pooncerelease+poacquireonce
>  
> +(*
> + * 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)
> diff --git a/tools/memory-model/litmus-tests/MP+porevlocks.litmus b/tools/memory-model/litmus-tests/MP+porevlocks.litmus
> index 90d011c34f33..bd68debfaa95 100644
> --- a/tools/memory-model/litmus-tests/MP+porevlocks.litmus
> +++ b/tools/memory-model/litmus-tests/MP+porevlocks.litmus
> @@ -1,5 +1,14 @@
>  C MP+porevlocks
>  
> +(*
> + * 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 relaasing a
> + * given lock), a CPU is not only guaranteed to see the accesses that other
> + * CPOs made while previously holding that lock, it are also guaranteed
> + * to see all prior accesses by those other CPUs.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y, spinlock_t *mylock)
> diff --git a/tools/memory-model/litmus-tests/MP+wmbonceonce+rmbonceonce.litmus b/tools/memory-model/litmus-tests/MP+wmbonceonce+rmbonceonce.litmus
> index 604ad41ea0c2..3d53ba138acd 100644
> --- a/tools/memory-model/litmus-tests/MP+wmbonceonce+rmbonceonce.litmus
> +++ b/tools/memory-model/litmus-tests/MP+wmbonceonce+rmbonceonce.litmus
> @@ -1,5 +1,11 @@
>  C MP+wmbonceonce+rmbonceonce
>  
> +(*
> + * 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)
> diff --git a/tools/memory-model/litmus-tests/R+mbonceonces.litmus b/tools/memory-model/litmus-tests/R+mbonceonces.litmus
> index e69b9e3e9436..4d64e547f1cd 100644
> --- a/tools/memory-model/litmus-tests/R+mbonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/R+mbonceonces.litmus
> @@ -1,5 +1,12 @@
>  C R+mbonceonces
>  
> +(*
> + * 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.  This test should be forbidden, but weaking either

s/weaking/weakening

(ispell suggests so, at least ...)

  Andrea


> + * of the barriers would cause the resulting test to be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/R+poonceonces.litmus b/tools/memory-model/litmus-tests/R+poonceonces.litmus
> index f7a12e00f82d..e75295b4e7c1 100644
> --- a/tools/memory-model/litmus-tests/R+poonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/R+poonceonces.litmus
> @@ -1,5 +1,11 @@
>  C R+poonceonces
>  
> +(*
> + * This is the unordered (via smp_mb()) version of one of the classic
> + * counterintuitive litmus tests that illustrates the effects of store
> + * propagation delays.  This test should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/S+poonceonces.litmus b/tools/memory-model/litmus-tests/S+poonceonces.litmus
> index d0d541c8ec7d..7fe16920a228 100644
> --- a/tools/memory-model/litmus-tests/S+poonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/S+poonceonces.litmus
> @@ -1,5 +1,13 @@
>  C S+poonceonces
>  
> +(*
> + * 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.  The answer is "of course not!",
> + * so this test should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/S+wmbonceonce+poacquireonce.litmus b/tools/memory-model/litmus-tests/S+wmbonceonce+poacquireonce.litmus
> index 1d292d0d6603..f78ce120863b 100644
> --- a/tools/memory-model/litmus-tests/S+wmbonceonce+poacquireonce.litmus
> +++ b/tools/memory-model/litmus-tests/S+wmbonceonce+poacquireonce.litmus
> @@ -1,5 +1,11 @@
>  C S+wmbonceonce+poacquireonce
>  
> +(*
> + * Can a smp_wmb(), instead of a release, and an acquire order a prior
> + * store against a subsequent store?  The answer should be "yes", so
> + * this test should be forbidden.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/SB+mbonceonces.litmus b/tools/memory-model/litmus-tests/SB+mbonceonces.litmus
> index b76caa5af1af..476542cd4a49 100644
> --- a/tools/memory-model/litmus-tests/SB+mbonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/SB+mbonceonces.litmus
> @@ -1,5 +1,12 @@
>  C SB+mbonceonces
>  
> +(*
> + * 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 read.  (Locking and RCU can also
> + * suffice, but not much else.)
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/SB+poonceonces.litmus b/tools/memory-model/litmus-tests/SB+poonceonces.litmus
> index c1797e03807e..40d519408ea6 100644
> --- a/tools/memory-model/litmus-tests/SB+poonceonces.litmus
> +++ b/tools/memory-model/litmus-tests/SB+poonceonces.litmus
> @@ -1,5 +1,11 @@
>  C SB+poonceonces
>  
> +(*
> + * 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 read.  This test should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x, int *y)
> diff --git a/tools/memory-model/litmus-tests/WRC+poonceonces+Once.litmus b/tools/memory-model/litmus-tests/WRC+poonceonces+Once.litmus
> index f5e7c92f61cc..0780a67cf3bd 100644
> --- a/tools/memory-model/litmus-tests/WRC+poonceonces+Once.litmus
> +++ b/tools/memory-model/litmus-tests/WRC+poonceonces+Once.litmus
> @@ -1,5 +1,11 @@
>  C WRC+poonceonces+Once
>  
> +(*
> + * This litmus test is an extension of the message-passing pattern, where
> + * the first write is moved to a separate process.  But because this test
> + * has no ordering at all, it should be allowed.
> + *)
> +
>  {}
>  
>  P0(int *x)
> diff --git a/tools/memory-model/litmus-tests/WRC+pooncerelease+rmbonceonce+Once.litmus b/tools/memory-model/litmus-tests/WRC+pooncerelease+rmbonceonce+Once.litmus
> index e3d0018025dd..070166d435e5 100644
> --- a/tools/memory-model/litmus-tests/WRC+pooncerelease+rmbonceonce+Once.litmus
> +++ b/tools/memory-model/litmus-tests/WRC+pooncerelease+rmbonceonce+Once.litmus
> @@ -1,5 +1,11 @@
>  C WRC+pooncerelease+rmbonceonce+Once
>  
> +(*
> + * 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)
> diff --git a/tools/memory-model/litmus-tests/Z6.0+pooncelock+poonceLock+pombonce.litmus b/tools/memory-model/litmus-tests/Z6.0+pooncelock+poonceLock+pombonce.litmus
> index 9c2cb53e6ef0..4d0a25665655 100644
> --- a/tools/memory-model/litmus-tests/Z6.0+pooncelock+poonceLock+pombonce.litmus
> +++ b/tools/memory-model/litmus-tests/Z6.0+pooncelock+poonceLock+pombonce.litmus
> @@ -1,5 +1,12 @@
>  C Z6.0+pooncelock+poonceLock+pombonce
>  
> +(*
> + * 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)
> diff --git a/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus b/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
> index c9a1f1a49ae1..8c723892716f 100644
> --- a/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
> +++ b/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
> @@ -1,5 +1,11 @@
>  C Z6.0+pooncelock+pooncelock+pombonce
>  
> +(*
> + * 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)
> diff --git a/tools/memory-model/litmus-tests/Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus b/tools/memory-model/litmus-tests/Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus
> index 25409a033514..8b0b1b3ca348 100644
> --- a/tools/memory-model/litmus-tests/Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus
> +++ b/tools/memory-model/litmus-tests/Z6.0+pooncerelease+poacquirerelease+mbonceonce.litmus
> @@ -1,5 +1,17 @@
>  C Z6.0+pooncerelease+poacquirerelease+mbonceonce
>  
> +(*
> + * 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)
>