Date: Wed, 29 Oct 2014 11:43:48 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
        fweisbec@gmail.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 9/9] rcu: Provide counterpart to
 rcu_dereference() for non-RCU situations
Message-ID: <20141029184348.GD5718@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20141028220944.GA26114@linux.vnet.ibm.com>
 <1414534202-27312-1-git-send-email-paulmck@linux.vnet.ibm.com>
 <1414534202-27312-9-git-send-email-paulmck@linux.vnet.ibm.com>
 <20141029105704.GY3337@twins.programming.kicks-ass.net>
 <20141029124203.GK5718@linux.vnet.ibm.com>
 <20141029191518.GA16312@redhat.com>
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On Wed, Oct 29, 2014 at 08:15:19PM +0100, Oleg Nesterov wrote:
> On 10/29, Paul E. McKenney wrote:
> >
> > On Wed, Oct 29, 2014 at 11:57:04AM +0100, Peter Zijlstra wrote:
> > > On Tue, Oct 28, 2014 at 03:10:02PM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > >
> > > > Although rcu_dereference() and friends can be used in situations where
> > > > object lifetimes are being managed by something other than RCU, the
> > > > resulting sparse and lockdep-RCU noise can be annoying.  This commit
> > > > therefore supplies a lockless_dereference(), which provides the
> > > > protection for dereferences without the RCU-related debugging noise.
> > > >
> > > > Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
> > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > > ---
> > >
> > > > +#define lockless_dereference(p) \
> > > > +({ \
> > > > +	typeof(p) _________p1 = ACCESS_ONCE(p); \
> > > > +	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> > > > +	(_________p1); \
> > > > +})
> > >
> > > Should we not have at least a single user along with this?
> >
> > And we do.  In fact, Al Viro has pulled this into his vfs.git tree and
> > so I will be dropping this patch in favor of his.
> 
> And it seems that most of smp_read_barrier_depends() users can be changed
> to use this helper.

Good point!  I guess I should have done this some time ago.  ;-)

							Thanx, Paul

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