linux-kernel - Re: [PATCH v2] sched: Make const-safe

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Date:   Wed, 14 Dec 2022 18:03:04 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [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.