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Date:   Mon, 10 Dec 2018 11:22:31 -0500 (EST)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
cc:     David Goldblatt <davidtgoldblatt@gmail.com>,
        <mathieu.desnoyers@efficios.com>,
        Florian Weimer <fweimer@redhat.com>, <triegel@redhat.com>,
        <libc-alpha@sourceware.org>, <andrea.parri@amarulasolutions.com>,
        <will.deacon@arm.com>, <peterz@infradead.org>,
        <boqun.feng@gmail.com>, <npiggin@gmail.com>, <dhowells@redhat.com>,
        <j.alglave@ucl.ac.uk>, <luc.maranget@inria.fr>, <akiyks@gmail.com>,
        <dlustig@nvidia.com>, <linux-arch@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Linux: Implement membarrier function
In-Reply-To: <20181206215405.GL4170@linux.ibm.com>
Message-ID: <Pine.LNX.4.44L0.1812101113430.1562-100000@iolanthe.rowland.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Thu, 6 Dec 2018, Paul E. McKenney wrote:

> Hello, David,
> 
> I took a crack at extending LKMM to accommodate what I think would
> support what you have in your paper.  Please see the very end of this
> email for a patch against the "dev" branch of my -rcu tree.
> 
> This gives the expected result for the following three litmus tests,
> but is probably deficient or otherwise misguided in other ways.  I have
> added the LKMM maintainers on CC for their amusement.  ;-)
> 
> Thoughts?

Since sys_membarrier() provides a heavyweight barrier comparable to 
synchronize_rcu(), the memory model should treat the two in the same 
way.  That's what this patch does.

The corresponding critical section would be any region of code bounded
by compiler barriers.  Since the LKMM doesn't currently handle plain
accesses, the effect is the same as if a compiler barrier were present
between each pair of instructions.  Basically, each instruction acts as
its own critical section.  Therefore the patch below defines memb-rscsi
as the trivial identity relation.  When plain accesses and compiler 
barriers are added to the memory model, a different definition will be 
needed.

This gives the correct results for the three C-Goldblat-memb-* litmus 
tests in Paul's email.

Alan

PS: The patch below is meant to apply on top of the SRCU patches, which
are not yet in the mainline kernel.


Index: usb-4.x/tools/memory-model/linux-kernel.bell
===================================================================
--- usb-4.x.orig/tools/memory-model/linux-kernel.bell
+++ usb-4.x/tools/memory-model/linux-kernel.bell
@@ -24,6 +24,7 @@ instructions RMW[{'once,'acquire,'releas
 enum Barriers = 'wmb (*smp_wmb*) ||
 		'rmb (*smp_rmb*) ||
 		'mb (*smp_mb*) ||
+		'memb (*sys_membarrier*) ||
 		'rcu-lock (*rcu_read_lock*)  ||
 		'rcu-unlock (*rcu_read_unlock*) ||
 		'sync-rcu (*synchronize_rcu*) ||
Index: usb-4.x/tools/memory-model/linux-kernel.cat
===================================================================
--- usb-4.x.orig/tools/memory-model/linux-kernel.cat
+++ usb-4.x/tools/memory-model/linux-kernel.cat
@@ -33,7 +33,7 @@ let mb = ([M] ; fencerel(Mb) ; [M]) |
 	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
 	([M] ; po ; [UL] ; (co | po) ; [LKW] ;
 		fencerel(After-unlock-lock) ; [M])
-let gp = po ; [Sync-rcu | Sync-srcu] ; po?
+let gp = po ; [Sync-rcu | Sync-srcu | Memb] ; po?
 
 let strong-fence = mb | gp
 
@@ -102,8 +102,10 @@ acyclic pb as propagation
  *)
 let rcu-gp = [Sync-rcu]		(* Compare with gp *)
 let srcu-gp = [Sync-srcu]
+let memb-gp = [Memb]
 let rcu-rscsi = rcu-rscs^-1
 let srcu-rscsi = srcu-rscs^-1
+let memb-rscsi = id
 
 (*
  * The synchronize_rcu() strong fence is special in that it can order not
@@ -119,15 +121,19 @@ let rcu-link = po? ; hb* ; pb* ; prop ;
  * the synchronize_srcu() and srcu_read_[un]lock() calls refer to the same
  * struct srcu_struct location.
  *)
-let rec rcu-fence = rcu-gp | srcu-gp |
+let rec rcu-fence = rcu-gp | srcu-gp | memb-gp |
 	(rcu-gp ; rcu-link ; rcu-rscsi) |
 	((srcu-gp ; rcu-link ; srcu-rscsi) & loc) |
+	(memb-gp ; rcu-link ; memb-rscsi) |
 	(rcu-rscsi ; rcu-link ; rcu-gp) |
 	((srcu-rscsi ; rcu-link ; srcu-gp) & loc) |
+	(memb-rscsi ; rcu-link ; memb-gp) |
 	(rcu-gp ; rcu-link ; rcu-fence ; rcu-link ; rcu-rscsi) |
 	((srcu-gp ; rcu-link ; rcu-fence ; rcu-link ; srcu-rscsi) & loc) |
+	(memb-gp ; rcu-link ; rcu-fence ; rcu-link ; memb-rscsi) |
 	(rcu-rscsi ; rcu-link ; rcu-fence ; rcu-link ; rcu-gp) |
 	((srcu-rscsi ; rcu-link ; rcu-fence ; rcu-link ; srcu-gp) & loc) |
+	(memb-rscsi ; rcu-link ; rcu-fence ; rcu-link ; memb-gp) |
 	(rcu-fence ; rcu-link ; rcu-fence)
 
 (* rb orders instructions just as pb does *)
Index: usb-4.x/tools/memory-model/linux-kernel.def
===================================================================
--- usb-4.x.orig/tools/memory-model/linux-kernel.def
+++ usb-4.x/tools/memory-model/linux-kernel.def
@@ -20,6 +20,7 @@ smp_store_mb(X,V) { __store{once}(X,V);
 smp_mb() { __fence{mb}; }
 smp_rmb() { __fence{rmb}; }
 smp_wmb() { __fence{wmb}; }
+smp_memb() { __fence{memb}; }
 smp_mb__before_atomic() { __fence{before-atomic}; }
 smp_mb__after_atomic() { __fence{after-atomic}; }
 smp_mb__after_spinlock() { __fence{after-spinlock}; }