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Subject[RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo
Date
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@amarulasolutions.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.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
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