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SubjectRe: [PATCH v2] sched: Make const-safe
On Wed, Dec 14, 2022 at 10:11:01AM +0100, Peter Zijlstra wrote:
> On Mon, Dec 12, 2022 at 02:49:46PM +0000, Matthew Wilcox (Oracle) wrote:
> > With a modified container_of() that preserves constness, the compiler
> > finds some pointers which should have been marked as const. task_of()
> > also needs to become const-preserving for the !FAIR_GROUP_SCHED case so
> > that cfs_rq_of() can take a const argument. No change to generated code.
> More const more better I suppose.. Thanks!
> Happen to have a sha for the container_of() commit handy?

There isn't one yet. Obviously we can't make container_of()
const-preserving until we've fixed all the places which would warn.
The diff I have in my tree looks like this:

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 1d898f9158b4..9416e6cc8c88 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -20,7 +20,10 @@
static_assert(__same_type(*(ptr), ((type *)0)->member) || \
__same_type(*(ptr), void), \
"pointer type mismatch in container_of()"); \
- ((type *)(__mptr - offsetof(type, member))); })
+ __mptr -= offsetof(type, member); \
+ _Generic(ptr, \
+ const typeof(*(ptr)) *: (const type *)__mptr, \
+ default: ((type *)__mptr)); })

* container_of_const - cast a member of a structure out to the containing
I have all of fs/ and net/ compiling cleanly now. There are a few
places which really need the const-removing properties, and I've made
those call a new macro called container_of_not_const(), but I don't
like that name.

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