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

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Date:   Tue, 24 Jan 2023 20:54:56 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        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 Tue, Jan 24, 2023 at 02:54:49PM -0800, Paul E. McKenney wrote:
> On Tue, Jan 24, 2023 at 05:35:33PM -0500, Alan Stern wrote:
> > Can you be more explicit?  Exactly what guarantees does the kernel 
> > implementation make that can't be expressed in LKMM?
> 
> I doubt that I will be able to articulate it very well, but here goes.
> 
> Within the Linux kernel, the rule for a given RCU "domain" is that if
> an event follows a grace period in pretty much any sense of the word,
> then that event sees the effects of all events in all read-side critical
> sections that began prior to the start of that grace period.
> 
> Here the senses of the word "follow" include combinations of rf, fr,
> and co, combined with the various acyclic and irreflexive relations
> defined in LKMM.

The LKMM says pretty much the same thing.  In fact, it says the event 
sees the effects of all events po-before the unlock of (not just inside) 
any read-side critical section that began prior to the start of the 
grace period.

> > And are these anything the memory model needs to worry about?
> 
> Given that several people, yourself included, are starting to use LKMM
> to analyze the Linux-kernel RCU implementations, maybe it does.
> 
> Me, I am happy either way.

Judging from your description, I don't think we have anything to worry 
about.

Alan