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

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Message-ID: <Y8WTXS73qTBpUzcI@rowland.harvard.edu>
Date:   Mon, 16 Jan 2023 13:11:41 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        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 Sun, Jan 15, 2023 at 08:23:29PM -0800, Paul E. McKenney wrote:
> On Sun, Jan 15, 2023 at 03:46:10PM -0500, Alan Stern wrote:
> > On Sun, Jan 15, 2023 at 10:10:52AM -0800, Paul E. McKenney wrote:
> > > On Sun, Jan 15, 2023 at 11:23:31AM -0500, Alan Stern wrote:
> > > > On Sat, Jan 14, 2023 at 09:15:10PM -0800, Paul E. McKenney wrote:
> > > > > What am I missing here?
> > > > 
> > > > I don't think you're missing anything.  This is a matter for Boqun or 
> > > > Luc; it must have something to do with the way herd treats the 
> > > > srcu_read_lock() and srcu_read_unlock() primitives.
> > > 
> > > It looks like we need something that tracks (data | rf)* between
> > > the return value of srcu_read_lock() and the second parameter of
> > > srcu_read_unlock().  The reason for rf rather than rfi is the upcoming
> > > srcu_down_read() and srcu_up_read().
> > 
> > Or just make herd treat srcu_read_lock(s) as an annotated equivalent of 
> > READ_ONCE(&s) and srcu_read_unlock(s, v) as an annotated equivalent of 
> > WRITE_ONCE(s, v).  But with some special accomodation to avoid 
> > interaction with the new carry-dep relation.
> 
> This is a modification to herd7 you are suggesting?  Otherwise, I am
> suffering a failure of imagination on how to properly sort it from the
> other READ_ONCE() and WRITE_ONCE() instances.

srcu_read_lock and srcu_read_unlock events would be distinguished from 
other marked loads and stores by belonging to the Srcu-lock and 
Srcu-unlock sets.  But I don't know whether this result can be 
accomplished just by modifying the .def file -- it might require changes 
to herd7.  (In fact, as far as I know there is no documentation at all 
for the double-underscore operations used in linux-kernel.def.  Hint 
hint!)

As mentioned earlier, we should ask Luc or Boqun.


> > > Or is there some better intermediate position that could be taken?
> > 
> > Do you mean go back to the current linux-kernel.bell?  The code you 
> > wrote above is different, since it prohibits nesting.
> 
> Not to the current linux-kernel.bell, but, as you say, making the change
> to obtain a better approximation by prohibiting nesting.

Why do you want to prohibit nesting?  Why would that be a better 
approximation?

Alan