linux-kernel - Re: [RFC][PATCH 7/7] sched: Replace rq::wake

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Message-ID: <20200605081028.GC2750@hirez.programming.kicks-ass.net>
Date:   Fri, 5 Jun 2020 10:10:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     tglx@...utronix.de, frederic@...nel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org, cai@....pw,
        mgorman@...hsingularity.net
Subject: Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list

On Thu, Jun 04, 2020 at 07:18:37AM -0700, Guenter Roeck wrote:
> On Tue, May 26, 2020 at 06:11:04PM +0200, Peter Zijlstra wrote:

> > +	BUILD_BUG_ON(offsetof(struct task_struct, wake_entry_type) - offsetof(struct task_struct, wake_entry) !=
> > +		     offsetof(struct __call_single_data, flags) - offsetof(struct __call_single_data, llist));
> > +
> 
> There is no guarantee in C that
> 
> 	type1 a;
> 	type2 b;
> 
> in two different data structures means that offsetof(b) - offsetof(a)
> is the same in both data structures unless attributes such as
> __attribute__((__packed__)) are used.

Do tell more; the alignment requirements and size of the types remains
the same, this resulting in different layout is unlikely.

I found this excellent quote on Hacker News this morning:

 "I think the attitude of compiler writers is a good reason to fix the
  spec so they can't keep ratfucking developers trying to get work done."

> As result, this does and will cause a variety of build errors depending
> on the compiler version and compile flags.

The only thing I can think of that's actually a problem is that retarded
struct randomization stuff.

Anyway, I'll move cleaning it up a little higher on the todo list.