From: Paul E. McKenney <paulmck@xxxxxxxxxx>
The xchg() and cmpxchg() functions are sometimes used to carry out RCU
updates. Unfortunately, this can result in sparse warnings for both
the old-value and new-value arguments, as well as for the return value.
The arguments can be dealt with using RCU_INITIALIZER():
old_p = xchg(&p, RCU_INITIALIZER(new_p));
But a sparse warning still remains due to assigning the __rcu pointer
returned from xchg to the (most likely) non-__rcu pointer old_p.
This commit therefore provides an unrcu_pointer() macro that strips
the __rcu. This macro can be used as follows:
old_p = unrcu_pointer(xchg(&p, RCU_INITIALIZER(new_p)));
Reported-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
include/linux/rcupdate.h | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index bd04f722714f..49f368c5d4ec 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -362,6 +362,20 @@ static inline void rcu_preempt_sleep_check(void) { }
#define rcu_check_sparse(p, space)
#endif /* #else #ifdef __CHECKER__ */
+/**
+ * unrcu_pointer - mark a pointer as not being RCU protected
+ * @p: pointer needing to lose its __rcu property
+ *
+ * Converts @p from an __rcu pointer to a __kernel pointer.
+ * This allows an __rcu pointer to be used with xchg() and friends.
+ */
+#define unrcu_pointer(p) \
+({ \
+ typeof(*p) *_________p1 = (typeof(*p) *__force)(p); \
+ rcu_check_sparse(p, __rcu); \
+ ((typeof(*p) __force __kernel *)(_________p1)); \
+})
+
#define __rcu_access_pointer(p, space) \
({ \
typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
