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

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Message-ID: <20210304190515.GS2696@paulmck-ThinkPad-P72>
Date:   Thu, 4 Mar 2021 11:05:15 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
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 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"?

							Thanx, Paul