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

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Message-ID: <Y8q+u09ynxnvjVi5@rowland.harvard.edu>
Date:   Fri, 20 Jan 2023 11:18:03 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     paulmck@...nel.org, Andrea Parri <parri.andrea@...il.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.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 Fri, Jan 20, 2023 at 11:13:00AM +0100, Jonas Oberhauser wrote:
> Perhaps we could say that reading an index without using it later is
> forbidden?
> 
> flag ~empty [Srcu-lock];data;rf;[~ domain(data;[Srcu-unlock])] as
> thrown-srcu-cookie-on-floor

We already flag locks that don't have a matching unlock.  I don't see 
any point in worrying about whatever else happens to the index.

> So if there is an srcu_down() that produces a cookie that is read by some
> read R, and R doesn't then pass that value into an srcu_up(), the
> srcu-warranty is voided.

No, it isn't.  As long as the value is passed to exactly one 
srcu_up_read(), it doesn't much matter what else you do with it.

Alan