[PATCH 3/3] container_of: remove local __mptr variable

[Vincent Mailhol]

container_of() can be called in a nested manner to retrieve the grand
parent structure as illustrated below:

	struct foo {
		int a;
	};

	struct bar {
		struct foo foo;
	};

	#define to_foo(a_ptr) container_of(a_ptr, struct foo, a)
	#define to_bar(a_ptr) container_of(to_foo(a_ptr), struct bar, foo)

The issue is that the above construct will cause __mptr, the local
variable of container_of(), to shadow itself because of the nested
call. This then triggers a warning in sparse and W=2 builds.

While this warning is benign, it still causes some overhead as proven by
below list of commits in which people made local workarounds:

  - commit 7eab14de73a8 ("mdio, phy: fix -Wshadow warnings triggered by
    nested container_of()")

  - commit 8d8c3131248d ("clk: define to_clk_regmap() as inline
    function")

  - commit bfb972c5e1cb ("IB/verbs: avoid nested container_of()")

  - commit 093adbcedf12 ("btrfs: switch helper macros to static inlines
    in sysfs.h")

  - commit c1d35dfa0f7d ("rt2x00: Fix sparse warning on nested
    container_of()")

(the list is probably not exhaustive).

As a matter of fact, the local variable __mptr is only used once in
container_of(). As such, it is not strictly needed. Inline that local
__mptr variable to remove once and for all the risk of variable
shadowing when nesting container_of() and prevent people from writing
further local fixes.

Signed-off-by: Vincent Mailhol <mailhol@xxxxxxxxxx>
---
 include/linux/container_of.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 68153170db32..28db38e9ee3e 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -17,11 +17,10 @@
  * Do not use container_of() in new code.
  */
 #define container_of(ptr, type, member) ({				\
-	void *__mptr = (void *)(ptr);					\
 	static_assert(__same_type(*(ptr), typeof_member(type, member)) || \
 		      __same_type(*(ptr), void),			\
 		      "pointer type mismatch in container_of()");	\
-	(type *)(__mptr - offsetof(type, member)); })
+	(type *)((void *)(ptr) - offsetof(type, member)); })
 
 /**
  * container_of_const - cast a member of a structure out to the containing

-- 
2.54.0