linux-kernel - [PATCH] locking/barriers: don't use sizeof(void) in lockless

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Date:   Fri, 26 Aug 2016 08:16:00 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     linux-kernel@...r.kernel.org
Cc:     "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, kbuild-all@...org,
        Johannes Berg <johannes.berg@...el.com>
Subject: [PATCH] locking/barriers: don't use sizeof(void) in lockless_dereference()

From: Johannes Berg <johannes.berg@...el.com>

My previous commit 112dc0c8069e ("locking/barriers: Suppress sparse
warnings in lockless_dereference()") caused sparse to complain that
(in radix-tree.h) we use sizeof(void) since that rcu_dereference()s
a void *.

Really, all we need is to have the expression *p in here somewhere
to make sure p is a pointer type, and sizeof(*p) was the thing that
came to my mind first to make sure that's done without really doing
anything at runtime.

Another thing I had considered was using typeof(*p), but obviously
we can't just declare a typeof(*p) variable either, since that may
end up being void. Declaring a variable as typeof(*p)* gets around
that, and still checks that typeof(*p) is valid, so do that. This
type construction can't be done for _________p1 because that will
actually be used and causes sparse address space warnings, so keep
a separate unused variable for it.

Reported-by: Fengguang Wu <fengguang.wu@...el.com>
Fixes: 112dc0c8069e ("locking/barriers: Suppress sparse warnings in lockless_dereference()")
Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
 include/linux/compiler.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 436aa4e42221..668569844d37 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -527,13 +527,14 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * object's lifetime is managed by something other than RCU.  That
  * "something other" might be reference counting or simple immortality.
  *
- * The seemingly unused size_t variable is to validate @p is indeed a pointer
- * type by making sure it can be dereferenced.
+ * The seemingly unused variable ___typecheck_p validates that @p is
+ * indeed a pointer type by using a pointer to typeof(*p) as the type.
+ * Taking a pointer to typeof(*p) again is needed in case p is void *.
  */
 #define lockless_dereference(p) \
 ({ \
 	typeof(p) _________p1 = READ_ONCE(p); \
-	size_t __maybe_unused __size_of_ptr = sizeof(*(p)); \
+	typeof(*(p)) *___typecheck_p __maybe_unused; \
 	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
 	(_________p1); \
 })
-- 
2.8.1