On Fri, Feb 15, 2008 at 04:40:24PM -0800, Andrew Morton wrote:
> On Wed, 13 Feb 2008 14:00:24 -0800
> "Paul E. McKenney" <[EMAIL PROTECTED]> wrote:
>
> > Hello!
> >
> > This is an updated version of the patch posted last November:
> >
> > http://archives.free.net.ph/message/20071201.003721.cd6ff17c.en.html
> >
> > This new version permits arguments with side effects, for example:
> >
> > rcu_assign_pointer(global_p, p++);
> >
> > and also verifies that the arguments are pointers, while still avoiding
> > the unnecessary memory barrier when assigning NULL to a pointer.
> > This memory-barrier avoidance means that rcu_assign_pointer() is now only
> > permitted for pointers (not array indexes), and so this version emits a
> > compiler warning if the first argument is not a pointer. I built a "make
> > allyesconfig" version on an x86 system, and received no such warnings.
> > If RCU is ever applied to array indexes, then the second patch in this
> > series should be applied, and the resulting rcu_assign_index() be used.
> >
> > Given the rather surprising history of subtlely broken implementations of
> > rcu_assign_pointer(), I took the precaution of generating a full set of
> > test cases and verified that memory barriers and compiler warnings were
> > emitted when required. I guess it is the simple things that get you...
> >
> > Signed-off-by: Paul E. McKenney <[EMAIL PROTECTED]>
> > ---
> >
> > rcupdate.h | 16 ++++++++++++----
> > 1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h
> > linux-2.6.24-rap/include/linux/rcupdate.h
> > --- linux-2.6.24/include/linux/rcupdate.h 2008-01-24 14:58:37.000000000
> > -0800
> > +++ linux-2.6.24-rap/include/linux/rcupdate.h 2008-02-13
> > 13:36:47.000000000 -0800
> > @@ -270,12 +270,20 @@ extern struct lockdep_map rcu_lock_map;
> > * structure after the pointer assignment. More importantly, this
> > * call documents which pointers will be dereferenced by RCU read-side
> > * code.
> > + *
> > + * Throws a compiler warning for non-pointer arguments.
> > + *
> > + * Does not insert a memory barrier for a NULL pointer.
> > */
> >
> > -#define rcu_assign_pointer(p, v) ({ \
> > - smp_wmb(); \
> > - (p) = (v); \
> > - })
> > +#define rcu_assign_pointer(p, v) \
> > + ({ \
> > + typeof(*p) *_________p1 = (v); \
> > + \
> > + if (!__builtin_constant_p(v) || (_________p1 != NULL)) \
> > + smp_wmb(); \
> > + (p) = _________p1; \
> > + })
> >
>
> umm...
>
> net/netfilter/core.c: In function 'nf_register_afinfo':
> net/netfilter/core.c:39: warning: initialization discards qualifiers from
> pointer target type
> net/netfilter/nf_log.c: In function 'nf_log_register':
> net/netfilter/nf_log.c:37: warning: initialization discards qualifiers from
> pointer target type
> net/netfilter/nf_queue.c: In function 'nf_register_queue_handler':
> net/netfilter/nf_queue.c:38: warning: initialization discards qualifiers from
> pointer target type
Hmmm... Netfilter compiles cleanly here. My guess is that your gcc
is more fastidious about const declarations. Could you please either
let me know what arch/gcc-settings you are using, or, alternatively,
see if the following patch fixes things up? The comparison against
NULL should at least emit warnings for non-pointer types -- not as
good as an error, but better than emitting bogus warnings.
So I guess I should stick with simple things like preemptable RCU instead
of the much more difficult task of outsmarting gcc...
Signed-off-by: Paul E. McKenney <[EMAIL PROTECTED]>
---
rcupdate.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- rcupdate.h.old 2008-02-15 17:18:50.000000000 -0800
+++ rcupdate.h 2008-02-15 17:18:52.000000000 -0800
@@ -176,7 +176,7 @@ struct rcu_head {
#define rcu_assign_pointer(p, v) \
({ \
- typeof(*p) *_________p1 = (v); \
+ typeof(p) _________p1 = (v); \
\
if (!__builtin_constant_p(v) || ((_________p1) != NULL)) \
smp_wmb(); \
--
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