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Subject: Re: [RFC][PATCH 7/7] sched: Replace rq::wake_list
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, frederic@kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org, cai@lca.pw,
        mgorman@techsingularity.net
References: <20200526161057.531933155@infradead.org>
 <20200526161908.129371594@infradead.org>
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From:   Guenter Roeck <linux@roeck-us.net>
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Date:   Fri, 5 Jun 2020 06:33:38 -0700
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Sender: linux-kernel-owner@vger.kernel.org
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On 6/5/20 1:10 AM, Peter Zijlstra wrote:
> 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 have not made the C standard. You point out yourself a possible explicit
culprit: struct randomization. That by itself shows that you can not rely
on two elements of different structures having the same alignment,
which is pretty much exactly what I said (and may explain why observing
the problem seemed to at least somewhat depend on the weather).

> 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."
> 

Qed.

Guenter

>> 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.
>