Date: Tue, 1 Aug 2017 06:35:11 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>, Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents
Reply-To: paulmck@linux.vnet.ibm.com
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 <20170726115328.2sxiitivlnlq64dk@hirez.programming.kicks-ass.net>
 <20170726124750.vktrn5zi2gmpzfru@tardis>
 <20170731090535.rjgnoewqg7mhzr55@hirez.programming.kicks-ass.net>
 <20170731110403.ou3zqsp3uviqorkz@tardis>
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On Tue, Aug 01, 2017 at 11:01:21AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 31, 2017 at 10:43:45AM -0700, Paul E. McKenney wrote:
> 
> > Why wouldn't the following have ACQUIRE semantics?
> > 
> > 	atomic_inc(&var);
> > 	smp_mb__after_atomic();
> > 
> > Is the issue that there is no actual value returned or some such?
> 
> Yes, so that the inc is a load-store, and thus there is a load, we loose
> the value.
> 
> But I see your point I think. Irrespective of still having the value,
> the ordering is preserved and nothing should pass across that.
> 
> > So if I have something like this, the assertion really can trigger?
> > 
> > 	WRITE_ONCE(x, 1);		atomic_inc(&y);
> > 	r0 = xchg_release(&y, 5);	smp_mb__after_atomic();
> > 					r1 = READ_ONCE(x);
> > 
> > 
> > 	WARN_ON(r0 == 0 && r1 == 0);
> > 
> > I must confess that I am not seeing why we would want to allow this
> > outcome.
> 
> No you are indeed quite right. I just wasn't creative enough. Thanks for
> the inspiration.

Whew!  You guys had me worried there for a bit.  ;-)

							Thanx, Paul