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

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Message-ID: <20151006210243.GX3910@linux.vnet.ibm.com>
Date:	Tue, 6 Oct 2015 14:02:43 -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,
	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 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.

							Thanx, Paul

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