Messages in this thread Patch in this message | | | Date | Tue, 20 Dec 2022 13:59:21 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] sched: Make const-safe |
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On Wed, Dec 14, 2022 at 06:03:04PM +0000, Matthew Wilcox wrote: > 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.
#define const_cast(T, exp) _Generic((exp), const T : (T)(exp), default: (exp))
perhaps? Then one can write something like:
struct task_struct *p = const_cast(struct task_struct *, constainer_of(node, struct task_struct *, run_node));
The repetition is a bit naf, but at least the construct is more generally useful.
(I really wish there was a qualifier stripping typeof() variant -- and yes, I know about the _Atomic thing).
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