linux-kernel - Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads

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Date:   Fri, 23 Jul 2021 15:29:37 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Manfred Spraul <manfred@...orfullife.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, parri.andrea@...il.com,
        will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com
Subject: Re: [PATCH memory-model 2/4] tools/memory-model: Add example for
 heuristic lockless reads

On Fri, Jul 23, 2021 at 05:03:47PM -0400, Alan Stern wrote:
> On Fri, Jul 23, 2021 at 01:28:11PM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 23, 2021 at 02:11:38PM -0400, Alan Stern wrote:
> > > In other words, if the second read races with the WRITE_ONCE, it needs 
> to
> > > get either the value before the write or the value after the write;
> > > nothing else will do because it isn't a heuristic here.  Fair point.
> > >
> > > >  (If the value changes immediately after being read, the fact that
> > > > ->f_lock is held prevents begin_global() from completing.)
> > >
> > > This seems like something worth explaining in the document.  That
> > > "IMPORTANT" comment doesn't really get the full point across.
> >
> > How about this comment instead?
> >
> >       /* This works even if data_race() returns nonsense. */
> 
> That's somewhat better.
> 
> 
> On Fri, Jul 23, 2021 at 01:32:48PM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 23, 2021 at 01:08:20PM -0400, Alan Stern wrote:
> > > This doesn't mention the reason for the acquire-release
> > > synchronization of global_flag.  It's needed because work done between
> > > begin_global() and end_global() can affect a foo structure without
> > > holding its private f_lock member, and we want all such work to be
> > > visible to other threads when they call do_something_locked() later.
> > 
> > Like this added paragraph at the end?
> > 
> > 	The smp_load_acquire() and smp_store_release() are required
> > 	because changes to a foo structure between calls to begin_global()
> > 	and end_global() are carried out without holding that structure's
> > 	->f_lock.  The smp_load_acquire() and smp_store_release()
> > 	ensure that the next invocation of do_something() from the call
> > 	to do_something_locked() that acquires that ->f_lock will see
> > 	those changes.
> 
> I'd shorten the last sentence:
> 
> 	The smp_load_acquire() and smp_store_release() ensure that the
> 	next invocation of do_something() from do_something_locked()
> 	will see those changes.

Sold!  ;-)

							Thanx, Paul