[tip: core/rcu] rcu: Create an unrcu_pointer() to remove __rcu from a pointer

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     76c8eaafe4f061f3790112842a2fbb297e4bea88
Gitweb:        https://git.kernel.org/tip/76c8eaafe4f061f3790112842a2fbb297e4bea88
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Wed, 21 Apr 2021 14:30:54 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 13 May 2021 09:13:23 -07:00

rcu: Create an unrcu_pointer() to remove __rcu from a pointer

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>
---
 include/linux/rcupdate.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 1199ffd..b071d02 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -363,6 +363,20 @@ 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); \