Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff()

[Josh Triplett <josh@joshtriplett.org>]

On Tue, Oct 06, 2015 at 10:31:52AM -0700, Paul E. McKenney wrote:
> 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@linux.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?

Ah, I see.  Per the example, you don't call this on an __rcu pointer
directly, only on a pointer you've already obtained from RCU after
giving it a lifetime of its own.  Nevermind.

> > >  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
> > > 
> > 
>