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:   Mon, 22 Oct 2018 19:09:52 +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 (10/21/18 11:09), Daniel Wang wrote:
> 
> Just got back from vacation. Thanks for the continued discussion. Just so
> I understand the current state. Looks like we've got a pretty good explanation
> of what's going on (though not completely sure), and backporting Steven's
> patches is still the way to go?

Up to -stable maintainers.

Note, with or without Steven's patch, the non-reentrable consoles are
still non-reentrable, so the deadlock is still there:

	spin_lock_irqsave(&port->lock, flags)
	 <NMI>
	  panic()
	   console_flush_on_panic()
	    spin_lock_irqsave(&port->lock, flags)		// deadlock


// And I wouldn't mind to have more reviews/testing on [1].


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 see that Sergey had sent an RFC series for similar things. Are those
> trying to solve the deadlock problem in a different way?

Umm, I wouldn't call it "another way". It turns non-reentrant serial
consoles to re-entrable ones. Did you test patch [1] from that series
on you environment, by the way?

[1] lkml.kernel.org/r/20181016050428.17966-2-sergey.senozhatsky@...il.com

	-ss