linux-kernel - Re: 4.14 backport request for dbdda842fe96f: "printk: Add console owner and waiter logic to load balance console writes"

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Date:   Tue, 2 Oct 2018 21:23:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Wang <wonderfly@...gle.com>
Cc:     Petr Mladek <pmladek@...e.com>, stable@...r.kernel.org,
        Alexander.Levin@...rosoft.com, akpm@...ux-foundation.org,
        byungchul.park@....com, dave.hansen@...el.com, hannes@...xchg.org,
        jack@...e.cz, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Mel Gorman <mgorman@...e.de>, mhocko@...nel.org, pavel@....cz,
        penguin-kernel@...ove.sakura.ne.jp, peterz@...radead.org,
        tj@...nel.org, torvalds@...ux-foundation.org, vbabka@...e.cz,
        Cong Wang <xiyou.wangcong@...il.com>,
        Peter Feiner <pfeiner@...gle.com>
Subject: Re: 4.14 backport request for dbdda842fe96f: "printk: Add console
 owner and waiter logic to load balance console writes"

On Tue, 2 Oct 2018 17:15:17 -0700
Daniel Wang <wonderfly@...gle.com> wrote:

> On Tue, Oct 2, 2018 at 1:42 AM Petr Mladek <pmladek@...e.com> wrote:
> >
> > Well, I still wonder why it helped and why you do not see it with 4.4.
> > I have a feeling that the console owner switch helped only by chance.
> > In fact, you might be affected by a race in
> > printk_safe_flush_on_panic() that was fixed by the commit:
> >
> > 554755be08fba31c7 printk: drop in_nmi check from printk_safe_flush_on_panic()
> >
> > The above one commit might be enough. Well, there was one more
> > NMI-related race that was fixed by:
> >
> > ba552399954dde1b printk: Split the code for storing a message into the log buffer
> > a338f84dc196f44b printk: Create helper function to queue deferred console handling
> > 03fc7f9c99c1e7ae printk/nmi: Prevent deadlock when accessing the main log buffer in NMI  
> 
> All of these commits already exist in 4.14 stable, since 4.14.68. The deadlock
> still exists even when built from 4.14.73 (latest tag) though. And cherrypicking
> dbdda842fe96 fixes it.
> 

I don't see the big deal of backporting this. The biggest complaints
about backports are from fixes that were added to late -rc releases
where the fixes didn't get much testing. This commit was added in 4.16,
and hasn't had any issues due to the design. Although a fix has been
added:

c14376de3a1 ("printk: Wake klogd when passing console_lock owner")

Also from 4.16, but nothing else according to searching for "Fixes"
tags.

-- Steve