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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, Andrew Morton <akpm@linux-foundation.org>,
	josh@joshtriplett.org, Thomas Gleixner <tglx@linutronix.de>,
	rostedt <rostedt@goodmis.org>,
	David Howells <dhowells@redhat.com>,
	edumazet@google.com, 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
Date: Thu, 25 Feb 2016 09:21:43 +0100	[thread overview]
Message-ID: <20160225082143.GZ6357@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160224214013.GF3522@linux.vnet.ibm.com>

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.

  parent reply	other threads:[~2016-02-25  8:21 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-24  5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores Paul E. McKenney
2016-02-24 21:12   ` Mathieu Desnoyers
2016-02-24 21:40     ` Paul E. McKenney
2016-02-25  6:41       ` Jianyu Zhan
2016-02-25 14:08         ` Paul E. McKenney
2016-02-25  8:21       ` Peter Zijlstra [this message]
2016-02-25 14:07         ` Paul E. McKenney
2016-02-25 14:48           ` Peter Zijlstra
2016-02-25 15:42             ` Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference() Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 08/14] documentation: Add alternative release-acquire outcome Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 09/14] documentation: Add documentation for RCU's major data structures Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier() Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention Paul E. McKenney
2016-02-24  5:00 ` [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example Paul E. McKenney

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