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 17:03:47 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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.

Alan