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

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Date:   Mon, 27 Feb 2023 21:13:01 +0100
From:   Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To:     Andrea Parri <parri.andrea@...il.com>,
        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



On 2/27/2023 8:40 PM, Andrea Parri wrote:
>> 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

Maybe not as unpopular as you think... :)
But either way IMHO it should be consistent; either take all the 
dependencies that are true and add them, or drop them all.
In the latter case, RCU should change to an acquire barrier. (also, one 
would have to deal with OOTA in some yet different way).

Generally my position is that unless there's a real-world benchmark with 
proven performance benefits of relying on dependency ordering, one 
should use an acquire barrier. I haven't yet met such a case, but maybe 
one of you has...

Best wishes,
jonas