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 17:15:17 -0700
From:   Daniel Wang <wonderfly@...gle.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     rostedt@...dmis.org, 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, 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.

>
> Best Regards,
> Petr



-- 
Best,
Daniel

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