linux-kernel - Re: [RFC v2 3/3] tools/memory-model: litmus: Add two tests for unlock(A)+lock(B) ordering

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Date:   Thu, 28 Oct 2021 12:11:29 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        Dan Lustig <dlustig@...dia.com>, Will Deacon <will@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Anvin <hpa@...or.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Stephane Eranian <eranian@...gle.com>, palmer@...belt.com,
        paul.walmsley@...ive.com, mpe@...erman.id.au,
        Jonathan Corbet <corbet@....net>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC v2 3/3] tools/memory-model: litmus: Add two tests for
 unlock(A)+lock(B) ordering

On Tue, Oct 26, 2021 at 09:01:00AM +0200, Peter Zijlstra wrote:
> On Mon, Oct 25, 2021 at 10:54:16PM +0800, Boqun Feng wrote:
> > diff --git a/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> > new file mode 100644
> > index 000000000000..955b9c7cdc7f
> > --- /dev/null
> > +++ b/tools/memory-model/litmus-tests/LB+unlocklockonceonce+poacquireonce.litmus
> > @@ -0,0 +1,33 @@
> > +C LB+unlocklockonceonce+poacquireonce
> > +
> > +(*
> > + * Result: Never
> > + *
> > + * If two locked critical sections execute on the same CPU, all accesses
> > + * in the first must execute before any accesses in the second, even if
> > + * the critical sections are protected by different locks.
> 
> One small nit; the above "all accesses" reads as if:
> 
> 	spin_lock(s);
> 	WRITE_ONCE(*x, 1);
> 	spin_unlock(s);
> 	spin_lock(t);
> 	r1 = READ_ONCE(*y);
> 	spin_unlock(t);
> 
> would also work, except of course that's the one reorder allowed by TSO.

I applied this series with Peter's Acked-by, and with the above comment
reading as follows:

+(*
+ * Result: Never
+ *
+ * If two locked critical sections execute on the same CPU, all accesses
+ * in the first must execute before any accesses in the second, even if the
+ * critical sections are protected by different locks.  The one exception
+ * to this rule is that (consistent with TSO) a prior write can be reordered
+ * with a later read from the viewpoint of a process not holding both locks.
+ *)

Thank you all!

							Thanx, Paul