linux-kernel - Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores

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Date:	Thu, 25 Feb 2016 09:21:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>, dipankar@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	josh@...htriplett.org, Thomas Gleixner <tglx@...utronix.de>,
	rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com,
	Oleg Nesterov <oleg@...hat.com>,
	bobby prani <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency
 and identical stores

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.