rcu: Create an unrcu_pointer() to remove __rcu from a pointer
authorPaul E. McKenney <paulmck@kernel.org>
Wed, 21 Apr 2021 21:30:54 +0000 (14:30 -0700)
committerPaul E. McKenney <paulmck@kernel.org>
Thu, 13 May 2021 16:13:23 +0000 (09:13 -0700)
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

index 1199ffd..b071d02 100644 (file)
@@ -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); \