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References: <20180927194601.207765-1-wonderfly@google.com> <20181001152324.72a20bea@gandalf.local.home>
 <CAJmjG29Jwn_1E5zexcm8eXTG=cTWyEr1gjSfSAS2fueB_V0tfg@mail.gmail.com> <20181002084225.6z2b74qem3mywukx@pathway.suse.cz>
In-Reply-To: <20181002084225.6z2b74qem3mywukx@pathway.suse.cz>
From:   Daniel Wang <wonderfly@google.com>
Date:   Tue, 2 Oct 2018 10:21:39 -0700
Message-ID: <CAJmjG28m_MoMAPZetVuQTod6CQm75DSvj1=kvGMopF01WgKAcA@mail.gmail.com>
Subject: Re: 4.14 backport request for dbdda842fe96f: "printk: Add console
 owner and waiter logic to load balance console writes"
To:     Petr Mladek <pmladek@suse.com>
Cc:     rostedt@goodmis.org, stable@vger.kernel.org,
        Alexander.Levin@microsoft.com, akpm@linux-foundation.org,
        byungchul.park@lge.com, dave.hansen@intel.com, hannes@cmpxchg.org,
        jack@suse.cz, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Mel Gorman <mgorman@suse.de>, mhocko@kernel.org, pavel@ucw.cz,
        penguin-kernel@i-love.sakura.ne.jp, peterz@infradead.org,
        tj@kernel.org, torvalds@linux-foundation.org, vbabka@suse.cz,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Peter Feiner <pfeiner@google.com>
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On Tue, Oct 2, 2018 at 1:42 AM Petr Mladek <pmladek@suse.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@gmail.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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