linux-kernel - Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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Date:   Tue, 1 Aug 2017 11:01:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC][PATCH v3]: documentation,atomic: Add new documents

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.