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:   Thu, 19 Apr 2018 16:22:22 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Thomas Backlund <tmb@...eia.org>,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        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>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Byungchul Park <byungchul.park@....com>,
        Tejun Heo <tj@...nel.org>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH AUTOSEL for 4.14 015/161] printk: Add console owner and
 waiter logic to load balance console writes

On Thu, Apr 19, 2018 at 04:05:45PM +0200, Jan Kara wrote:
> On Thu 19-04-18 15:59:43, Greg KH wrote:
> > On Thu, Apr 19, 2018 at 02:41:33PM +0300, Thomas Backlund wrote:
> > > Den 16-04-2018 kl. 19:19, skrev Sasha Levin:
> > > > On Mon, Apr 16, 2018 at 12:12:24PM -0400, Steven Rostedt wrote:
> > > > > On Mon, 16 Apr 2018 16:02:03 +0000
> > > > > Sasha Levin <Alexander.Levin@...rosoft.com> wrote:
> > > > > 
> > > > > > One of the things Greg is pushing strongly for is "bug compatibility":
> > > > > > we want the kernel to behave the same way between mainline and stable.
> > > > > > If the code is broken, it should be broken in the same way.
> > > > > 
> > > > > 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.
> > > > 
> > > 
> > > Yeah, but _intentionally_ breaking existing setups to stay "bug compatible"
> > > _is_ a _regression_ you _really_ _dont_ want in a stable
> > > supported distro. Because end-users dont care about upstream breaking
> > > stuff... its the distro that takes the heat for that...
> > > 
> > > Something "already broken" is not a regression...
> > > 
> > > As distro maintainer that means one now have to review _every_ patch that
> > > carries "AUTOSEL", follow all the mail threads that comes up about it, then
> > > track if it landed in -stable queue, and read every response and possible
> > > objection to all patches in the -stable queue a second time around... then
> > > check if it still got included in final stable point relase and then either
> > > revert them in distro kernel or go track down all the follow-up fixes
> > > needed...
> > > 
> > > Just to avoid being "bug compatible with master"
> > 
> > I've done this "bug compatible" "breakage" more than the AUTOSEL stuff
> > has in the past, so you had better also be reviewing all of my normal
> > commits as well :)
> > 
> > Anyway, we are trying not to do this, but it does, and will,
> > occasionally happen.
> 
> Sure, that's understood. So this was just misunderstanding. Sasha's
> original comment really sounded like "bug compatibility" with current
> master is desirable and that made me go "Are you serious?" as well...

As I said before in this thread, yes, sometimes I do this on purpose.

As an specific example, see a recent bluetooth patch that caused a
regression on some chromebook devices.  The chromeos developers
rightfully complainied, and I left the commit in there to provide the
needed "leverage" on the upstream developers to fix this properly.
Otherwise if I had reverted the stable patch, when people move to a
newer kernel version, things break, and no one remembers why.

I also wrote a long response as to _why_ I do this, and even though it
does happen, why it still is worth taking the stable updates.  Please
see the archives for the full details.  I don't want to duplicate this
again here.

thanks,

greg k-h