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Date: Thu, 25 Feb 2016 15:48:38 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
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        dvhart@linux.intel.com, fweisbec@gmail.com,
        Oleg Nesterov <oleg@redhat.com>, bobby prani <bobby.prani@gmail.com>
Subject: Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency
 and identical stores
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On Thu, Feb 25, 2016 at 06:07:03AM -0800, Paul E. McKenney wrote:
> Still bad wording...
> 
> It hasn't actually moved anything over the barrier().  It has instead
> moved both the barrier() and the WRITE_ONCE(b, p) to precede the "if
> (q)".  Mathieu mentioned this over IRC yesterday, and I queue a change
> so that the paragraph now reads as follows:
> 
>   (*) If both legs of the "if" statement begin with identical stores to
>       the same variable, then those stores must be ordered, either by
>       preceding both of them with smp_mb() or by using smp_store_release()
>       to carry out the stores.  Please note that it is -not- sufficient
>       to use barrier() at beginning of each leg of the "if" statement
>       because, as shown by the example above, optimizing compilers can
>       destroy the control dependency while respecting the letter of the
>       barrier() law.
> 
> Does hat help?

Maybe.. I still feel the compiler should not do this; but I'm having a
hard time explaining why.