linux-kernel - Re: [PATCH] tools/memory-model: Document herd7 (internal) representation

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Date: Wed, 5 Jun 2024 11:31:51 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Andrea Parri <parri.andrea@...il.com>
Cc: will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
  npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
  luc.maranget@...ia.fr, paulmck@...nel.org, akiyks@...il.com,
  dlustig@...dia.com, joel@...lfernandes.org, linux-kernel@...r.kernel.org,
  linux-arch@...r.kernel.org, hernan.poncedeleon@...weicloud.com,
  jonas.oberhauser@...weicloud.com
Subject: Re: [PATCH] tools/memory-model: Document herd7 (internal)
 representation

On Mon, May 27, 2024 at 10:07:06AM +0200, Andrea Parri wrote:
> > It turns out the problem lies in the way lock.cat tries to calculate the 
> > rf relation for RU events (a spin_is_locked() that returns False).  The 
> > method it uses amounts to requiring that such events must read from the 
> > lock's initial value or an LU event (a spin_unlock()) in a different 
> > thread.  This clearly is wrong, and glaringly so in this litmus test 
> > since there are no other threads!
> > 
> > A patch to fix the problem and reorganize the code a bit for greater 
> > readability is below.  I'd appreciate it if people could try it out on 
> > various locking litmus tests in our archives.
> 
> Thanks for the quick solution, Alan.  The results from our archives look
> good.

Here's a much smaller patch, suitable for the -stable kernels.  It fixes 
the bug without doing the larger code reorganization (which will go into 
a separate patch).  Can you test this one?

Alan



Index: usb-devel/tools/memory-model/lock.cat
===================================================================
--- usb-devel.orig/tools/memory-model/lock.cat
+++ usb-devel/tools/memory-model/lock.cat
@@ -102,19 +102,19 @@ let rf-lf = rfe-lf | rfi-lf
  * within one of the lock's critical sections returns False.
  *)
 
-(* rfi for RU events: an RU may read from the last po-previous UL *)
-let rfi-ru = ([UL] ; po-loc ; [RU]) \ ([UL] ; po-loc ; [LKW] ; po-loc)
-
-(* rfe for RU events: an RU may read from an external UL or the initial write *)
-let all-possible-rfe-ru =
-	let possible-rfe-ru r =
+(*
+ * rf for RU events: an RU may read from an external UL or the initial write,
+ * or from the last po-previous UL
+ *)
+let all-possible-rf-ru =
+	let possible-rf-ru r =
 		let pair-to-relation p = p ++ 0
-		in map pair-to-relation (((UL | IW) * {r}) & loc & ext)
-	in map possible-rfe-ru RU
+		in map pair-to-relation ((((UL | IW) * {r}) & loc & ext) |
+			(((UL * {r}) & po-loc) \ ([UL] ; po-loc ; [LKW] ; po-loc)))
+	in map possible-rf-ru RU
 
 (* Generate all rf relations for RU events *)
-with rfe-ru from cross(all-possible-rfe-ru)
-let rf-ru = rfe-ru | rfi-ru
+with rf-ru from cross(all-possible-rf-ru)
 
 (* Final rf relation *)
 let rf = rf | rf-lf | rf-ru