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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Manfred Spraul <manfred@colorfullife.com>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	kernel-team@fb.com, mingo@kernel.org, parri.andrea@gmail.com,
	will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
	npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
	luc.maranget@inria.fr, akiyks@gmail.com
Subject: Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads
Date: Fri, 23 Jul 2021 15:29:37 -0700	[thread overview]
Message-ID: <20210723222937.GM4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210723210347.GA53526@rowland.harvard.edu>

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

  reply	other threads:[~2021-07-23 22:29 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-21 21:07 [PATCH kcsan 0/8] KCSAN updates for v5.15 Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 1/8] kcsan: Improve some Kconfig comments Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 2/8] kcsan: Remove CONFIG_KCSAN_DEBUG Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 3/8] kcsan: Introduce CONFIG_KCSAN_STRICT Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 4/8] kcsan: Reduce get_ctx() uses in kcsan_found_watchpoint() Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 5/8] kcsan: Rework atomic.h into permissive.h Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 6/8] kcsan: Print if strict or non-strict during init Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 7/8] kcsan: permissive: Ignore data-racy 1-bit value changes Paul E. McKenney
2021-07-21 21:08 ` [PATCH kcsan 8/8] kcsan: Make strict mode imply interruptible watchers Paul E. McKenney
2021-07-21 21:10 ` [PATCH memory-model 1/4] tools/memory-model: Make read_foo_diagnostic() more clearly diagnostic Paul E. McKenney
2021-07-21 21:10 ` [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads Paul E. McKenney
2021-07-23  2:08   ` Alan Stern
2021-07-23  6:52     ` Manfred Spraul
2021-07-23 13:05       ` Alan Stern
2021-07-23 13:57         ` Manfred Spraul
2021-07-23 16:30         ` Paul E. McKenney
2021-07-23 17:08           ` Alan Stern
2021-07-23 20:32             ` Paul E. McKenney
2021-07-23 21:03               ` Alan Stern
2021-07-23 22:29                 ` Paul E. McKenney [this message]
2021-07-23 16:24     ` Paul E. McKenney
2021-07-23 16:59       ` Alan Stern
2021-07-23 17:30         ` Paul E. McKenney
2021-07-23 18:11           ` Alan Stern
2021-07-23 20:28             ` Paul E. McKenney
2021-07-28 17:40   ` [PATCH v2 " Paul E. McKenney
2021-07-21 21:10 ` [PATCH memory-model 3/4] tools/memory-model: Heuristics using data_race() must handle all values Paul E. McKenney
2021-07-21 21:10 ` [PATCH memory-model 4/4] tools/memory-model: Document data_race(READ_ONCE()) Paul E. McKenney

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