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 10:21:39 -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.

So do I. I don't think Steven had the deadlock in mind when working on
that patch, but with that patch and that patch alone, the deadlock
disappeared.

> 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

Thanks for the pointer. Let me test this out.

> 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
>
> Best Regards,
> Petr


On Tue, Oct 2, 2018 at 1:17 AM Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> wrote:

> Sorry, I'm missing something here. Steven's patch deals with lockups and
> I understand why you want to backport the patch set; but console output
> deadlock on panic() is another thing.

Understood. But it did fix the deadlock for me and without it I hit the deadlock
pretty consistently. Let me test the patch Petr pointed out and will update
here.

-- 
Best,
Daniel

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