locking/barriers: Don't use sizeof(void) in lockless_dereference()

My previous commit:

  112dc0c806 ("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@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kbuild-all@01.org
Fixes: 112dc0c806 ("locking/barriers: Suppress sparse warnings in lockless_dereference()")
Link: http://lkml.kernel.org/r/1472192160-4049-1-git-send-email-johannes@sipsolutions.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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); \
 })