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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: 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,
	Manfred Spraul <manfred@colorfullife.com>
Subject: Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads
Date: Fri, 23 Jul 2021 10:30:10 -0700	[thread overview]
Message-ID: <20210723173010.GI4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210723165947.GA46562@rowland.harvard.edu>

On Fri, Jul 23, 2021 at 12:59:47PM -0400, Alan Stern wrote:
> On Fri, Jul 23, 2021 at 09:24:31AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 22, 2021 at 10:08:46PM -0400, Alan Stern wrote:
> 
> > > > +	void do_something_locked(struct foo *fp)
> > > > +	{
> > > > +		bool gf = true;
> > > > +
> > > > +		/* IMPORTANT: Heuristic plus spin_lock()! */
> > > > +		if (!data_race(global_flag)) {
> > > > +			spin_lock(&fp->f_lock);
> > > > +			if (!smp_load_acquire(&global_flag)) {
> 
> > > > +	void begin_global(void)
> > > > +	{
> > > > +		int i;
> > > > +
> > > > +		spin_lock(&global_lock);
> > > > +		WRITE_ONCE(global_flag, true);
> > > 
> > > Why does this need to be WRITE_ONCE?  It still races with the first read 
> > > of global_flag above.
> > 
> > But also with the smp_load_acquire() of global_flag, right?
> 
> What I'm curious about is why, given these two races, you notate one of 
> them by changing a normal write to WRITE_ONCE and you notate the other 
> by changing a normal read to a data_race() read.  Why not handle them 
> both the same way?

Because the code can tolerate the first read returning complete nonsense,
but needs the value from the second read to be exact at that point in
time.  (If the value changes immediately after being read, the fact that
->f_lock is held prevents begin_global() from completing.)

							Thanx, Paul

  reply	other threads:[~2021-07-23 17:30 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
2021-07-23 16:24     ` Paul E. McKenney
2021-07-23 16:59       ` Alan Stern
2021-07-23 17:30         ` Paul E. McKenney [this message]
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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