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 14:11:38 -0400 [thread overview]
Message-ID: <20210723181138.GA48833@rowland.harvard.edu> (raw)
In-Reply-To: <20210723173010.GI4397@paulmck-ThinkPad-P17-Gen-1>
On Fri, Jul 23, 2021 at 10:30:10AM -0700, Paul E. McKenney wrote:
> 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.
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.
Alan
next prev parent reply other threads:[~2021-07-23 18:11 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
2021-07-23 18:11 ` Alan Stern [this message]
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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