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SubjectRe: [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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