linux-kernel - Re: [PATCH AUTOSEL for 4.14 015/161] printk: Add console owner and waiter logic to load balance console writes

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Date:   Mon, 16 Apr 2018 17:23:30 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     Pavel Machek <pavel@....cz>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>, Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Byungchul Park <byungchul.park@....com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH AUTOSEL for 4.14 015/161] printk: Add console owner and
 waiter logic to load balance console writes

On Mon, Apr 16, 2018 at 07:06:04PM +0200, Pavel Machek wrote:
>On Mon 2018-04-16 16:37:56, Sasha Levin wrote:
>> On Mon, Apr 16, 2018 at 12:30:19PM -0400, Steven Rostedt wrote:
>> >On Mon, 16 Apr 2018 16:19:14 +0000
>> >Sasha Levin <Alexander.Levin@...rosoft.com> wrote:
>> >
>> >> >Wait! What does that mean? What's the purpose of stable if it is as
>> >> >broken as mainline?
>> >>
>> >> This just means that if there is a fix that went in mainline, and the
>> >> fix is broken somehow, we'd rather take the broken fix than not.
>> >>
>> >> In this scenario, *something* will be broken, it's just a matter of
>> >> what. We'd rather have the same thing broken between mainline and
>> >> stable.
>> >
>> >Honestly, I think that removes all value of the stable series. I
>> >remember when the stable series were first created. People were saying
>> >that it wouldn't even get to more than 5 versions, because the bar for
>> >backporting was suppose to be very high. Today it's just a fork of the
>> >kernel at a given version. No more features, but we will be OK with
>> >regressions. I'm struggling to see what the benefit of it is suppose to
>> >be?
>>
>> It's not "OK with regressions".
>>
>> Let's look at a hypothetical example: You have a 4.15.1 kernel that has
>> a broken printf() behaviour so that when you:
>>
>> 	pr_err("%d", 5)
>>
>> Would print:
>>
>> 	"Microsoft Rulez"
>>
>> Bad, right? So you went ahead and fixed it, and now it prints "5" as you
>> might expect. But alas, with your patch, running:
>>
>> 	pr_err("%s", "hi!")
>>
>> Would show a cat picture for 5 seconds.
>>
>> Should we take your patch in -stable or not? If we don't, we're stuck
>> with the original issue while the mainline kernel will behave
>> differently, but if we do - we introduce a new regression.
>
>Of course not.
>
>- It must be obviously correct and tested.
>
>If it introduces new bug, it is not correct, and certainly not
>obviously correct.

As you might have noticed, we don't strictly follow the rules.

Take a look at the whole PTI story as an example. It's way more than 100
lines, it's not obviously corrent, it fixed more than 1 thing, and so
on, and yet it went in -stable!

Would you argue we shouldn't have backported PTI to -stable?