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:   Fri, 9 Nov 2018 15:47:40 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Daniel Wang <wonderfly@...gle.com>
Cc:     sergey.senozhatsky.work@...il.com, Petr Mladek <pmladek@...e.com>,
        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 (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