[tip:locking/urgent] locking/barriers: Don't use sizeof(void) in lockless_dereference()

[tip-bot for Johannes Berg]

Commit-ID:  d7127b5e5fa0551be21b86640f1648b224e36d43
Gitweb:     http://git.kernel.org/tip/d7127b5e5fa0551be21b86640f1648b224e36d43
Author:     Johannes Berg <johannes.berg@xxxxxxxxx>
AuthorDate: Fri, 26 Aug 2016 08:16:00 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 5 Sep 2016 11:50:42 +0200

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

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@xxxxxxxxx>
Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E . McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kbuild-all@xxxxxx
Fixes: 112dc0c8069e ("locking/barriers: Suppress sparse warnings in lockless_dereference()")
Link: http://lkml.kernel.org/r/1472192160-4049-1-git-send-email-johannes@xxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 436aa4e..6685698 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); \
 })