linux-kernel - Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer

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Message-ID: <20151007072227.GV2881@worktop.programming.kicks-ass.net>
Date:	Wed, 7 Oct 2015 09:22:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.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 10/13] rcu: Add rcu_pointer_handoff()

On Tue, Oct 06, 2015 at 02:02:43PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 06, 2015 at 10:27:41PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 06, 2015 at 09:13:45AM -0700, Paul E. McKenney wrote:
> > >  /**
> > > + * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
> > > + * @p: The pointer to hand off
> > > + *
> > > + * This is simply an identity function, but it documents where a pointer
> > > + * is handed off from RCU to some other synchronization mechanism, for
> > > + * example, reference counting or locking.  In C11, it would map to
> > > + * kill_dependency().  It could be used as follows:
> > > + *
> > > + *	rcu_read_lock();
> > > + *	p = rcu_dereference(gp);
> > > + *	long_lived = is_long_lived(p);
> > > + *	if (long_lived) {
> > > + *		if (!atomic_inc_not_zero(p->refcnt))
> > > + *			long_lived = false;
> > > + *		else
> > > + *			p = rcu_pointer_handoff(p);
> > > + *	}
> > > + *	rcu_read_unlock();
> > > + */
> > > +#define rcu_pointer_handoff(p) (p)
> > 
> > Will you actually be using this? It seems a tad pointless to add if you
> > don't.
> 
> Some of the LLVM guys believe that they can diagnose RCU pointer leaks
> if this is used.  But yes, it does need to be used.

The thing is, I'm not convinced this is a 'sane' interface. Its _far_
too easy to forget. It doesn't make any kind of sense either, which is
part of why its hard to remember.
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