netdev - [PATCH RFC bpf-next 1/4] rcu: Create an unrcu_pointer() to remove _

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Message-ID: <161917591669.102337.2073449001446955820.stgit@toke.dk>
Date:   Fri, 23 Apr 2021 13:05:16 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH RFC bpf-next 1/4] rcu: Create an unrcu_pointer() to remove
 __rcu from a pointer

From: Paul E. McKenney <paulmck@...nel.org>

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@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 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); \