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.
next prev 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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