linux-kernel - [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

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Date:   Tue, 19 Feb 2019 23:57:37 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>
Subject: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

Remove this subtle (and, AFAICT, unused) ordering: we can add it back,
if necessary, but let us not encourage people to rely on this thing.

For example, the following "exists" clause can be satisfied with this
change:

C dep-rfi

{ }

P0(int *x, int *y)
{
	WRITE_ONCE(*x, 1);
	smp_store_release(y, 1);
}

P1(int *x, int *y, int *z)
{
	int r0;
	int r1;
	int r2;

	r0 = READ_ONCE(*y);
	WRITE_ONCE(*z, r0);
	r1 = smp_load_acquire(z);
	r2 = READ_ONCE(*x);
}

exists (1:r0=1 /\ 1:r2=0)

Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Will Deacon <will.deacon@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Jade Alglave <j.alglave@....ac.uk>
Cc: Luc Maranget <luc.maranget@...ia.fr>
Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc: Akira Yokosawa <akiyks@...il.com>
Cc: Daniel Lustig <dlustig@...dia.com>
---
 tools/memory-model/Documentation/explanation.txt | 28 ------------------------
 tools/memory-model/linux-kernel.cat              |  2 +-
 2 files changed, 1 insertion(+), 29 deletions(-)

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index 68caa9a976d0c..965e11744d090 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/tools/memory-model/Documentation/explanation.txt
@@ -1019,34 +1019,6 @@ section for more details).  The kernel includes a workaround for this
 problem when the loads come from READ_ONCE(), and therefore the LKMM
 includes address dependencies to loads in the ppo relation.
 
-On the other hand, dependencies can indirectly affect the ordering of
-two loads.  This happens when there is a dependency from a load to a
-store and a second, po-later load reads from that store:
-
-	R ->dep W ->rfi R',
-
-where the dep link can be either an address or a data dependency.  In
-this situation we know it is possible for the CPU to execute R' before
-W, because it can forward the value that W will store to R'.  But it
-cannot execute R' before R, because it cannot forward the value before
-it knows what that value is, or that W and R' do access the same
-location.  However, if there is merely a control dependency between R
-and W then the CPU can speculatively forward W to R' before executing
-R; if the speculation turns out to be wrong then the CPU merely has to
-restart or abandon R'.
-
-(In theory, a CPU might forward a store to a load when it runs across
-an address dependency like this:
-
-	r1 = READ_ONCE(ptr);
-	WRITE_ONCE(*r1, 17);
-	r2 = READ_ONCE(*r1);
-
-because it could tell that the store and the second load access the
-same location even before it knows what the location's address is.
-However, none of the architectures supported by the Linux kernel do
-this.)
-
 Two memory accesses of the same location must always be executed in
 program order if the second access is a store.  Thus, if we have
 
diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index 8dcb37835b613..6b9e3bb4e397f 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -62,7 +62,7 @@ let dep = addr | data
 let rwdep = (dep | ctrl) ; [W]
 let overwrite = co | fr
 let to-w = rwdep | (overwrite & int)
-let to-r = addr | (dep ; rfi)
+let to-r = addr ; [R]
 let fence = strong-fence | wmb | po-rel | rmb | acq-po
 let ppo = to-r | to-w | fence | (po-unlock-rf-lock-po & int)
 
-- 
2.7.4