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container_of: remove local __mptr variable

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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 7eab14de73 ("mdio, phy: fix -Wshadow warnings triggered by
    nested container_of()")

  - commit 8d8c313124 ("clk: define to_clk_regmap() as inline
    function")

  - commit bfb972c5e1 ("IB/verbs: avoid nested container_of()")

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

  - commit c1d35dfa0f ("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@kernel.org>
Link: https://patch.msgid.link/20260714-containerof_refactor-v1-3-b5c31164d2ad@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Vincent Mailhol
2026-07-14 20:18:03 +02:00
committed by Greg Kroah-Hartman
parent 1ead62bc1b
commit f9e7a75648
1 changed files with 1 additions and 2 deletions
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3
include/linux/container_of.h
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@@ -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