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

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Message-ID: <20151006173152.GI3910@linux.vnet.ibm.com>
Date:	Tue, 6 Oct 2015 10:31:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, peterz@...radead.org, 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 10:21:28AM -0700, Josh Triplett wrote:
> On Tue, Oct 06, 2015 at 09:13:45AM -0700, Paul E. McKenney wrote:
> > This commit adds an rcu_pointer_handoff() that is intended to mark
> > situations where a structure's protection transitions from RCU to some
> > other mechanism (locking, reference counting, whatever).  These markings
> > should allow external tools to more easily spot bugs involving leaking
> > pointers out of RCU read-side critical sections.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Shouldn't this expect the __rcu address space on the pointer, and cast
> away the __rcu with __force?

I do not believe so, given that the __rcu was already removed by a preceding
rcu_dereference().  Or am I missing something?

							Thanx, Paul

> >  include/linux/rcupdate.h | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 6c3ceceb6148..587eb057e2fa 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -813,6 +813,28 @@ static inline void rcu_preempt_sleep_check(void)
> >  #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
> >  
> >  /**
> > + * 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)
> > +
> > +/**
> >   * rcu_read_lock() - mark the beginning of an RCU read-side critical section
> >   *
> >   * When synchronize_rcu() is invoked on one CPU while other CPUs
> > -- 
> > 2.5.2
> > 
> 

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