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Date: Fri, 9 Nov 2018 15:47:40 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: 4.14 backport request for dbdda842fe96f: "printk: Add console
 owner and waiter logic to load balance console writes"
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To: Daniel Wang <wonderfly@google.com>
Cc: sergey.senozhatsky.work@gmail.com, Petr Mladek <pmladek@suse.com>, 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>

On (11/01/18 09:05), Daniel Wang wrote:
> > Another deadlock scenario could be the following one:
> >
> >         printk()
> >          console_trylock()
> >           down_trylock()
> >            raw_spin_lock_irqsave(&sem->lock, flags)
> >             <NMI>
> >              panic()
> >               console_flush_on_panic()
> >                console_trylock()
> >                 raw_spin_lock_irqsave(&sem->lock, flags)        // deadlock
> >
> > There are no patches addressing this one at the moment. And it's
> > unclear if you are hitting this scenario.
> 
> I am not sure, but Steven's patches did make the deadlock I saw go away...

You certainly can find cases when "busy spin on console_sem owner" logic
can reduce some possibilities.

But spin_lock(&lock); NMI; spin_lock(&lock); code is still in the kernel.

> A little swamped by other things lately but I'll run a test with it.
> If it works, would you recommend taking your patch alone

Let's first figure out if it works.

	-ss