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Subject[tip: core/rcu] rcu: Create an unrcu_pointer() to remove __rcu from a pointer
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@kernel.org>
AuthorDate: Wed, 21 Apr 2021 14:30:54 -07:00
Committer: Paul E. McKenney <paulmck@kernel.org>
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@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
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); \
\
 
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