linux-kernel - Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

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Message-ID: <20230226184926.GA3213523@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 26 Feb 2023 10:49:26 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org, urezki@...il.com,
        quic_neeraju@...cinc.com, frederic@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

On Sat, Feb 25, 2023 at 07:03:11PM -0800, Paul E. McKenney wrote:
> On Sat, Feb 25, 2023 at 09:29:51PM -0500, Alan Stern wrote:
> > On Sat, Feb 25, 2023 at 05:01:10PM -0800, Paul E. McKenney wrote:
> > > A few other oddities:
> > > 
> > > litmus/auto/C-LB-Lww+R-OC.litmus
> > > 
> > > 	Both versions flag a data race, which I am not seeing.	It appears
> > > 	to me that P1's store to u0 cannot happen unless P0's store
> > > 	has completed.  So what am I missing here?
> > 
> > The LKMM doesn't believe that a control or data dependency orders a 
> > plain write after a marked read.  Hence in this test it thinks that P1's 
> > store to u0 can happen before the load of x1.  I don't remember why we 
> > did it this way -- probably we just wanted to minimize the restrictions 
> > on when plain accesses can execute.  (I do remember the reason for 
> > making address dependencies induce order; it was so RCU would work.)
> > 
> > The patch below will change what the LKMM believes.  It eliminates the 
> > positive outcome of the litmus test and the data race.  Should it be 
> > adopted into the memory model?
> 
> Excellent question!
> 
> As noted separately, I was conflating the C++ memory model and LKMM.
> 
> > > litmus/auto/C-LB-Lrw+R-OC.litmus
> > > litmus/auto/C-LB-Lww+R-Oc.litmus
> > > litmus/auto/C-LB-Lrw+R-Oc.litmus
> > > litmus/auto/C-LB-Lrw+R-A+R-Oc.litmus
> > > litmus/auto/C-LB-Lww+R-A+R-OC.litmus
> > > 
> > > 	Ditto.  (There are likely more.)
> > 
> > I haven't looked at these but they're probably similar.
> 
> Let me give this patch a go and see what it does.

And it operates as expected, converting Sometimes/data-race results
into Never.

Leaving the question of whether that is what we need.  ;-)

							Thanx, Paul