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Date: Thu, 25 Feb 2016 09:21:43 +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>,
        Lai Jiangshan <jiangshanlai@gmail.com>, dipankar@in.ibm.com,
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        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.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 Wed, Feb 24, 2016 at 01:40:13PM -0800, Paul E. McKenney wrote:
> > > +  (*) 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,
> > > +      as optimizing compilers do not necessarily respect barrier()
> > > +      in this case.

> Let's take the example, replace barrier() with smp_mb(), and see what
> happens:
> 
> 	q = READ_ONCE(a);
> 	if (q) {
> 		smp_mb();
> 		WRITE_ONCE(b, p);
> 		do_something();
> 	} else {
> 		smp_mb();
> 		WRITE_ONCE(b, p);
> 		do_something_else();
> 	}

Why would an optimizing compiler be allowed to lift _anything_ over a
barrier() ? Isn't that a bug?

I thought the whole point of barrier() was to tell the compiler to not
do such things.