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From: Alan Stern <stern@rowland.harvard.edu>
To: "Paul E. McKenney" <paulmck@kernel.org>
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 12:59:47 -0400	[thread overview]
Message-ID: <20210723165947.GA46562@rowland.harvard.edu> (raw)
In-Reply-To: <20210723162431.GF4397@paulmck-ThinkPad-P17-Gen-1>

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?

Alan

  reply	other threads:[~2021-07-23 16:59 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 [this message]
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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