linux-kernel - Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

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Message-ID: <Y7XojCwWskMDEkRS@rowland.harvard.edu>
Date:   Wed, 4 Jan 2023 15:58:52 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

On Wed, Jan 04, 2023 at 04:37:14PM +0100, Andrea Parri wrote:
> Sounds good to me too.  I'm trying to remember why we went for the LKW
> event to model smp_mb__after_unlock_lock() (as opposed to the LKR event,
> as suggested above/in po-unlock-lock-po).

I don't remember either, but with the LKR event it would be awkward to 
include the co part of (co | po) in the smp_mb__after_unlock_lock() 
definition.  You'd have to write something like ((co? ; rf) | po).

Aside from that, I don't think using LKR vs. LKW makes any difference.

>  Anyway, I currently see no 
> issue with the above (we know that LKW and LKR come paired), and I think
> it's good to merge the two notions of "unlock-lock pair" if possible.

Indeed.  It also would eliminate questions about why po-unlock-lock-po 
doesn't include the co term.

Alan