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Message-ID: <1570539989.5576.295.camel@lca.pw>
Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()
From:   Qian Cai <cai@lca.pw>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Petr Mladek <pmladek@suse.com>, akpm@linux-foundation.org,
        sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org,
        peterz@infradead.org, linux-mm@kvack.org,
        john.ogness@linutronix.de, david@redhat.com,
        linux-kernel@vger.kernel.org
Date:   Tue, 08 Oct 2019 09:06:29 -0400
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On Tue, 2019-10-08 at 14:39 +0200, Michal Hocko wrote:
> On Tue 08-10-19 08:00:43, Qian Cai wrote:
> > 
> > 
> > > On Oct 8, 2019, at 6:39 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > 
> > > Have you actually triggered any real deadlock? With a zone->lock in
> > > place it would be pretty clear with hard lockups detected.
> > 
> > Yes, I did trigger here and there, and those lockdep splats are
> > especially useful to figure out why.
> 
> Can you provide a lockdep splat from an actual deadlock please? I am
> sorry but your responses tend to be really cryptic and I never know when
> you are talking about actual deadlocks and lockdep splats. I have asked
> about the former several times never receiving a specific answer.

It is very time-consuming to confirm a lockdep splat is 100% matching a deadlock
giving that it is not able to reproduce on will yet, so when I did encounter a
memory offline deadlock where "echo offline > memory/state" just hang, but there
is no hard lockup probably because the hard lockup detector did not work
properly for some reasons or it keep trying to acquire a spin lock that only
keep the CPU 100%.