On Tue, Feb 12, 2008 at 08:57:14AM +0000, Jarek Poplawski wrote:
> On 12-02-2008 02:16, David Miller wrote:
> > From: Stephen Hemminger <[EMAIL PROTECTED]>
> > Date: Mon, 11 Feb 2008 16:59:54 -0800
> >
> > linux-kernel added to CC:, any change to generic kernel infrastructure
> > should be posted there
> >
> >> Eliminate warnings when rcu_assign_pointer is used with unsigned long.
> >> It is reasonable to use RCU with non-pointer values so allow it for general
> >> use. Add a comment to explain the if test.
> >>
> >> Signed-off-by: Stephen Hemminger <[EMAIL PROTECTED]>
> >> ---
> >> include/linux/rcupdate.h | 13 +++++++------
> >> 1 files changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> >> index 37a642c..c44ac87 100644
> >> --- a/include/linux/rcupdate.h
> >> +++ b/include/linux/rcupdate.h
> >> @@ -172,14 +172,15 @@ struct rcu_head {
> >> * structure after the pointer assignment. More importantly, this
> >> * call documents which pointers will be dereferenced by RCU read-side
> >> * code.
> >> + *
> >> + * If value is the NULL (constant 0), then no barrier is needed.
> >> */
> >>
> >> -#define rcu_assign_pointer(p, v) \
> >> - ({ \
> >> - if (!__builtin_constant_p(v) || \
> >> - ((v) != NULL)) \
> >> - smp_wmb(); \
> >> - (p) = (v); \
> >> +#define rcu_assign_pointer(p, v) \
> >> + ({ \
> >> + if (!(__builtin_constant_p(v) && v)) \
>
> ...But, "If value is the NULL (constant 0)" we have:
>
> if (!(1 && NULL != 0)) ==> if (!(0)) and the barrier is used?!
"All programmers are blind, especially me."
You are right, Jarek. I ran this through gcc, and the following
comes close:
#define rcu_assign_pointer(p, v) \
({ \
if (!__builtin_constant_p(v) || (v)) \
smp_wmb(); \
(p) = (v); \
})
But I am concerned about the following case:
rcu_assign_pointer(global_index, 0);
. . .
x = global_array[rcu_dereference(global_index)];
Since arrays have a zero-th element, we would really want a memory
barrier in this case.
So how about leaving the index-unfriendly version of rcu_assign_pointer()
and adding an rcu_assign_index() as follows?
Thanx, Paul
Signed-off-by: Paul E. McKenney <[EMAIL PROTECTED]>
---
rcupdate.h | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h
linux-2.6.24-rai/include/linux/rcupdate.h
--- linux-2.6.24/include/linux/rcupdate.h 2008-01-24 14:58:37.000000000
-0800
+++ linux-2.6.24-rai/include/linux/rcupdate.h 2008-02-12 08:04:59.000000000
-0800
@@ -278,6 +278,24 @@ extern struct lockdep_map rcu_lock_map;
})
/**
+ * rcu_assign_index - assign (publicize) a index of a newly
+ * initialized array elementg that will be dereferenced by RCU
+ * read-side critical sections. Returns the value assigned.
+ *
+ * Inserts memory barriers on architectures that require them
+ * (pretty much all of them other than x86), and also prevents
+ * the compiler from reordering the code that initializes the
+ * structure after the index assignment. More importantly, this
+ * call documents which indexes will be dereferenced by RCU read-side
+ * code.
+ */
+
+#define rcu_assign_index(p, v) ({ \
+ smp_wmb(); \
+ (p) = (v); \
+ })
+
+/**
* synchronize_sched - block until all CPUs have exited any non-preemptive
* kernel code sequences.
*
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