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

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Date:   Mon, 27 Feb 2023 20:40:00 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Jonas Oberhauser <jonas.oberhauser@...wei.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

> 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?

(Unpopular opinion I know,) it should drop dependencies ordering, not
add/promote it.

  Andrea