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Date: Mon, 22 Jan 2018 19:28:57 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup
Message-ID: <20180122102857.GC403@jagdpanzerIV>
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To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Tejun Heo <tj@kernel.org>, Petr Mladek <pmladek@suse.com>, akpm@linux-foundation.org, linux-mm@kvack.org, Cong Wang <xiyou.wangcong@gmail.com>, Dave Hansen <dave.hansen@intel.com>, Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>, Peter Zijlstra <peterz@infradead.org>, Linus Torvalds <torvalds@linux-foundation.org>, Jan Kara <jack@suse.cz>, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, rostedt@home.goodmis.org, Byungchul Park <byungchul.park@lge.com>, Pavel Machek <pavel@ucw.cz>, linux-kernel@vger.kernel.org

On (01/22/18 17:56), Sergey Senozhatsky wrote:
[..]
> Assume the following,

But more importantly we are missing another huge thing - console_unlock().

Suppose:

	console_lock();
	<< preemption >>
						printk
						printk
						..
						printk
	console_unlock()
	 for (;;) {
		call_console_drivers()
		   dump_stack
		   queue IRQ work

		IRQ work >>
		   flush_printk_safe
		   printk_deferred()
		   ...
		   printk_deferred()
		<< iret
	 }

This should explode: sleepable console_unlock() may reschedule,
printk_safe flush bypasses recursion checks.

	-ss

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