linux-kernel - Re: [PATCH tip/core/rcu 9/9] rcu: Provide counterpart to rcu

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Message-ID: <20141029124203.GK5718@linux.vnet.ibm.com>
Date:	Wed, 29 Oct 2014 05:42:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 9/9] rcu: Provide counterpart to
 rcu_dereference() for non-RCU situations

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@...ux.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@...IV.linux.org.uk>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.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.

							Thanx, Paul

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