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

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Message-ID: <Y8LbeLflWLyivOz9@rowland.harvard.edu>
Date:   Sat, 14 Jan 2023 11:42:32 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@...wei.com>
Cc:     "paulmck@...nel.org" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "parri.andrea" <parri.andrea@...il.com>, 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 13, 2023 at 02:55:34PM +0000, Jonas Oberhauser wrote:
> I think the whole rcu-order topic can be summarized as the 'one rule': "if a grace period happens before a rcsc-unlock, it must also happen before the rcsc -lock, and analogously if  rcsc-lock happens before a grace period, the rcsc-unlock also happens before the grace period" . 

There is more to it than that, as I mentioned earlier.  A complete
description can be found the explanation.txt document; it says:

	For any critical section C and any grace period G, at least
	one of the following statements must hold:

(1)	C ends before G does, and in addition, every store that
	propagates to C's CPU before the end of C must propagate to
	every CPU before G ends.

(2)	G starts before C does, and in addition, every store that
	propagates to G's CPU before the start of G must propagate
	to every CPU before C starts.

Alan