linux-kernel - Re: [PATCH tip/core/rcu 31/48] rcu: define _

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Date:	Tue, 4 May 2010 16:07:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com,
	Christopher Li <sparse@...isli.org>
Subject: Re: [PATCH tip/core/rcu 31/48] rcu: define __rcu address space
 modifier for sparse

On Tue, May 04, 2010 at 10:58:19PM +0200, Arnd Bergmann wrote:
> On Tuesday 04 May 2010 22:19:41 Paul E. McKenney wrote:
> > This patch defines an __rcu annotation that permits sparse to check for
> > correct use of RCU-protected pointers.  If a pointer that is annotated
> > with __rcu is accessed directly (as opposed to via rcu_dereference(),
> > rcu_assign_pointer(), or one of their variants), sparse can be made
> > to complain.  To enable such complaints, use the new default-disabled
> > CONFIG_SPARSE_RCU_POINTER kernel configuration option.  Please note that
> > these sparse complaints are intended to be a debugging aid, -not- a
> > code-style-enforcement mechanism.
> 
> To add more background, I was thinking that it might make sense to
> always leave the address space attribute in place but to make part
> part of the checking optional.
> 
> The idea would be that we always make sure that an __rcu annotated
> pointer cannot be dereferenced or cast directly, while we would
> only complain about non-annotated pointers being passed to rcu_dereference
> and rcu_assign_pointer if CONFIG_SPARSE_RCU_POINTER is set.
> 
> Most of the work I had spent on my tree was about fixing all the
> false positives from that, but more work would be needed to get
> a clean build from it even with the modified CONFIG_SPARSE_RCU_POINTER
> disabled. Since you managed to find the real bugs and fix them,
> your series by itself is probably more useful than the full set
> that I originally had.

Find thus far -- actually fixing the bugs is still on my list.  ;-)

> > +/*
> > + * Helper functions for rcu_dereference_check(), rcu_dereference_protected()
> > + * and rcu_assign_pointer().  Some of these could be folded into their
> > + * callers, but they are left separate in order to ease introduction of
> > + * multiple flavors of pointers to match the multiple flavors of RCU
> > + * (e.g., __rcu_bh, * __rcu_sched, and __srcu), should this make sense in
> > + * the future.
> > + */
> > +#define __rcu_access_pointer(p, space) \
> > +	({ \
> > +		typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
> > +		(void) (((typeof (*p) space *)p) == p); \
> > +		((typeof(*p) __force __kernel *)(_________p1)); \
> > +	})
> 
> Do you have specific plans to add these (__rcu_bh etc) back in the future,
> or do you just want to leave the options open?

No specific plans at the moment.  But having the underlying plumbing all
in one place greatly improves the readability.

> Anyway, good to see that you found your way through my patches and got them
> into shape. 

Thank -you- for making this happen!!!

							Thanx, Paul
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