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Subject[PATCH RFC tip/core/rcu 01/23] rcu: add an rcu_dereference_index_check()
Date
The sparse RCU-pointer checking relies on type magic that dereferences
the pointer in question. This does not work if the pointer is in fact
an array index. This commit therefore supplies a new RCU API that
omits the sparse checking to continue to support rcu_dereference()
on integers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/rcupdate.h | 33 +++++++++++++++++++++++++++++++++
1 files changed, 33 insertions(+), 0 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 2f9e56c..3be0ad7 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -560,4 +560,37 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
}
#endif /* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */

+#ifndef CONFIG_PROVE_RCU
+#define __do_rcu_dereference_check(c) do { } while (0)
+#endif /* #ifdef CONFIG_PROVE_RCU */
+
+#define __rcu_dereference_index_check(p, c) \
+ ({ \
+ typeof(p) _________p1 = ACCESS_ONCE(p); \
+ __do_rcu_dereference_check(c); \
+ smp_read_barrier_depends(); \
+ (_________p1); \
+ })
+
+/**
+ * rcu_dereference_index_check() - rcu_dereference for indices with debug checking
+ * @p: The pointer to read, prior to dereferencing
+ * @c: The conditions under which the dereference will take place
+ *
+ * Similar to rcu_dereference_check(), but omits the sparse checking.
+ * This allows rcu_dereference_index_check() to be used on integers,
+ * which can then be used as array indices. Attempting to use
+ * rcu_dereference_check() on an integer will give compiler warnings
+ * because the sparse address-space mechanism relies on dereferencing
+ * the RCU-protected pointer. Dereferencing integers is not something
+ * that even gcc will put up with.
+ *
+ * Note that this function does not implicitly check for RCU read-side
+ * critical sections. If this function gains lots of uses, it might
+ * make sense to provide versions for each flavor of RCU, but it does
+ * not make sense as of early 2010.
+ */
+#define rcu_dereference_index_check(p, c) \
+ __rcu_dereference_index_check((p), (c))
+
#endif /* __LINUX_RCUPDATE_H */
--
1.7.0.6


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