linux-kernel - Re: [PATCH 1/3] LKMM: Add litmus test for RCU GP guarantee where updater frees object

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Date:   Fri, 20 Mar 2020 16:15:16 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     linux-kernel@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>, linux-arch@...r.kernel.org,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/3] LKMM: Add litmus test for RCU GP guarantee where
 updater frees object

On Fri, Mar 20, 2020 at 10:59:55AM -0400, Alan Stern wrote:
> On Fri, 20 Mar 2020, Joel Fernandes (Google) wrote:
> 
> > This adds an example for the important RCU grace period guarantee, which
> > shows an RCU reader can never span a grace period.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  .../litmus-tests/RCU+sync+free.litmus         | 40 +++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> >  create mode 100644 tools/memory-model/litmus-tests/RCU+sync+free.litmus
> > 
> > diff --git a/tools/memory-model/litmus-tests/RCU+sync+free.litmus b/tools/memory-model/litmus-tests/RCU+sync+free.litmus
> > new file mode 100644
> > index 0000000000000..c4682502dd296
> > --- /dev/null
> > +++ b/tools/memory-model/litmus-tests/RCU+sync+free.litmus
> > @@ -0,0 +1,40 @@
> > +C RCU+sync+free
> > +
> > +(*
> > + * Result: Never
> > + *
> 
> The following comment needs some rewriting.  The grammar is somewhat
> awkward and a very important "not" is missing.
> 
> > + * This litmus test demonstrates that an RCU reader can never see a write after
> > + * the grace period, if it saw writes that happen before the grace period.
> 
> An RCU reader can never see a write that follows a grace period if it
> did _not_ see writes that precede the grace period.

Yes, you are right. I will change your wording to 'did not see *all* writes
that precede'.

> >  This
> > + * is a typical pattern of RCU usage, where the write before the grace period
> > + * assigns a pointer, and the writes after destroy the object that the pointer
> > + * points to.
> 
> ... that the pointer used to point to.

Will fix.

> > + *
> > + * This guarantee also implies, an RCU reader can never span a grace period and
> > + * is an important RCU grace period memory ordering guarantee.
> 
> Unnecessary comma, and it is not clear what "This" refers to.  The 
> whole sentence should be phrased differently:
> 
> 	This is one implication of the RCU grace-period guarantee,
> 	which says (among other things) that an RCU reader cannot span 
> 	a grace period.

Your wording is better, will use that.

thanks,

 - Joel