linux-kernel - Re: XDP socket rings, and LKMM litmus tests

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Date:   Thu, 4 Mar 2021 16:27:53 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        parri.andrea@...il.com, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
        joel@...lfernandes.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: XDP socket rings, and LKMM litmus tests

On Thu, Mar 04, 2021 at 11:05:15AM -0800, Paul E. McKenney wrote:
> On Thu, Mar 04, 2021 at 10:35:24AM -0500, Alan Stern wrote:
> > On Wed, Mar 03, 2021 at 09:04:07PM -0800, Paul E. McKenney wrote:
> > > On Wed, Mar 03, 2021 at 10:21:01PM -0500, Alan Stern wrote:
> > > > On Wed, Mar 03, 2021 at 02:03:48PM -0800, Paul E. McKenney wrote:
> > > > > On Wed, Mar 03, 2021 at 03:22:46PM -0500, Alan Stern wrote:
> > 
> > > > > > >  And I cannot immediately think of a situation where
> > > > > > > this approach would break that would not result in a data race being
> > > > > > > flagged.  Or is this yet another failure of my imagination?
> > > > > > 
> > > > > > By definition, an access to a local variable cannot participate in a 
> > > > > > data race because all such accesses are confined to a single thread.
> > > > > 
> > > > > True, but its value might have come from a load from a shared variable.
> > > > 
> > > > Then that load could have participated in a data race.  But the store to 
> > > > the local variable cannot.
> > > 
> > > Agreed.  My thought was that if the ordering from the initial (non-local)
> > > load mattered, then that initial load must have participated in a
> > > data race.  Is that true, or am I failing to perceive some corner case?
> > 
> > Ordering can matter even when no data race is involved.  Just think
> > about how much of the memory model is concerned with ordering of
> > marked accesses, which don't participate in data races unless there is
> > a conflicting plain access somewhere.
> 
> Fair point.  Should I have instead said "then that initial load must
> have run concurrently with a store to that same variable"?

I'm losing track of the point you were originally trying to make.

Does ordering matter when there are no conflicting accesses?  Sure.  
Consider this:

	A: r1 = READ_ONCE(x);
	B: WRITE_ONCE(y, r1);
	   smp_wmb();
	C: WRITE_ONCE(z, 1);

Even if there are no other accesses to y at all (let alone any 
conflicting ones), the mere existence of B forces A to be ordered before 
C, and this is easily detectable by a litmus test.

Alan