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Date: Wed, 21 Sep 2016 13:41:56 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
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Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Tejun Heo <tj@kernel.org>, Jiri Kosina <jkosina@suse.com>
Subject: Re: [Ksummit-discuss] [TECH TOPIC] asynchronous printk
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Hello,

a small addition to [1]

On (07/19/16 12:47), Sergey Senozhatsky wrote:
> What I have on my list is:
> - synchronous printk()
> - synchronous console_unlock()
> - console semaphore

Petr Mladek came up with a nice summary [2] of the problems that we
discussed (with a small update from my side [3]). So I would like to
add one more issue to the list, that is, however, not related to async
printk: recursive printk calls. There is a tiny set of cases when
printk can correctly handle recursive printk calls, the rest lead to
deadlock. Those problems, may be, not commonly seen, we had two or three
reports via the public mailing list in the past year; and I've seen 4-5
reports in internal bug tracker (not all of those errors were due to
incorrect printk usage. An unfortunate ARM imprecise abort [4] delivered
while the CPU is in printk->console_unlock->try_to_wake_up under the p->pi_lock
is enough to deadlock the system).

There a several ideas how to try to address those issues that I'm aware
of [5],[6],[7] and, I'm quite sure, even more ideas that I'm not aware of.
Would be interesting to discuss it.

[1] https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2016-July/002740.html
[2] https://marc.info/?l=linux-kernel&m=147263639304079
[3] https://marc.info/?l=linux-kernel&m=147264799407718&w=2
[4] http://infocenter.arm.com/help/topic/com.arm.doc.faqs/14809.html
[5] https://marc.info/?l=linux-kernel&m=147158843319944
[6] https://marc.info/?l=linux-kernel&m=147183933104154
[7] https://marc.info/?l=linux-kernel&m=145440084915204

> The list of potential attendees
> (not in any particular order, most likely incomplete)
> 
> Jan Kara
> Petr Mladek
> Andrew Morton
> Jiri Kosina
> Tejun Heo
> Hannes Reinecke
> Viresh Kumar
> Steven Rostedt
> Sergey Senozhatsky

	-ss