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

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Message-ID: <20230118174138.GB2948950@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 18 Jan 2023 09:41:38 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     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 Wed, Jan 18, 2023 at 11:03:35AM -0500, Alan Stern wrote:
> On Tue, Jan 17, 2023 at 09:17:04PM -0800, Paul E. McKenney wrote:
> > On Tue, Jan 17, 2023 at 09:15:15PM -0500, Alan Stern wrote:
> > > Maybe we don't.  Please test the patch below; I think it will do what 
> > > you want -- and it doesn't rule out nesting.
> > 
> > It works like a champ on manual/kernel/C-srcu*.litmus in the litmus
> > repository on github, good show and thank you!!!
> > 
> > I will make more tests, and am checking this against the rest of the
> > litmus tests in the repo, but in the meantime would you be willing to
> > have me add your Signed-off-by?
> 
> I'll email a real patch submission in the not-too-distant future, 
> assuming you don't find any problems with the new code.

Sounds good!

The current state is that last night's testing found a difference only
for C-srcu-nest-5.litmus, in which case your version gives the correct
answer and mainline is wrong.  There were a couple of broken tests, which
I fixed and a test involving spin_unlock_wait(), which is at this point
perma-broken due to the Linux kernel no longer having such a thing.
(Other than its re-introduction into i915, but they define it as a
spin_lock_irq() followed by a spin_unlock_irq(), so why worry?)
There were also a few timeouts.

I intend to run the longer tests overnight.

I have not yet come up with a good heuristic to auto-classify
automatically generated tests involving SRCU, so I cannot justify making
you wait on me to get my act together on that.

							Thanx, Paul