Date:   Mon, 23 Jan 2023 10:55:17 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     paulmck@kernel.org, Andrea Parri <parri.andrea@gmail.com>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
Message-ID: <Y86t5ST3TcUl/rQd@rowland.harvard.edu>
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On Mon, Jan 23, 2023 at 12:48:42PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/21/2023 6:36 PM, Alan Stern wrote:
> > On Fri, Jan 20, 2023 at 10:41:14PM +0100, Jonas Oberhauser wrote:
> > > 
> > > On 1/20/2023 5:18 PM, Alan Stern wrote:
> > > > 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.
> > > Of course, but as you know this is completely orthogonal.
> > Yeah, okay.  It doesn't hurt to add this check, but the check isn't
> > complete.  For example, it won't catch the invalid usage here:
> > 
> > P0(srcu_struct *ss)
> > {
> > 	int r1, r2;
> > 
> > 	r1 = srcu_read_lock(ss);
> > 	srcu_read_unlock(&ss, r1);
> > 	r2 = srcu_read_lock(ss);
> > 	srcu_read_unlock(&ss, r2);
> > }
> > 
> > exists (~0:r1=0:r2)
> > 
> > On the other hand, how often will people make this sort of mistake in
> > their litmus tests?  My guess is not very.
> I currently don't care too much about the incorrect usage of herd (by
> inspecting some final state incorrectly), only incorrect usage in the code.

I'm inclined to add this check to the memory model.  Would you prefer to 
submit it yourself as a separate patch?  Or are you happy to have it 
merged with my patch, and if so, do you have a final, preferred form for 
the check?

Alan