linux-kernel - Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

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Message-ID: <20180116051622.GB13731@jagdpanzerIV>
Date:   Tue, 16 Jan 2018 14:16:22 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On (01/15/18 09:51), Petr Mladek wrote:
> On Sat 2018-01-13 16:31:00, Sergey Senozhatsky wrote:
> > On (01/12/18 13:55), Petr Mladek wrote:
> > [..]
> > > > I'm not fixing console_unlock(), I'm fixing printk(). BTW, all my
> > > > kernels are CONFIG_PREEMPT (I'm a RT guy), my mind thinks more about
> > > > PREEMPT kernels than !PREEMPT ones.
> > > 
> > > I would say that the patch improves also console_unlock() but only in
> > > non-preemttive context.
> > > 
> > > By other words, it makes console_unlock() finite in preemptible context
> > > (limited by buffer size). It might still be unlimited in
> > > non-preemtible context.
> > 
> > could you elaborate a bit?
> 
> Ah, I am sorry, I swapped the conditions. I meant that
> console_unlock() is finite in non-preemptible context.

by the way. just for the record,

probably there is a way for us to have a task printing more than
O(logbuf) even in non-preemptible context.

	CPU0

	vprintk_emit()
	 preempt_disable()
	  console_unlock()
	  {
	   for (;;) {
                printk_safe_enter_irqsave()
	        call_console_drivers();
	        printk_safe_exit_irqrestore()

	<< IRQ >>
		dump_stack()
		 printk()->log_store()
		 ....
		 printk()->log_store()
	<< iret >>
	   }
	  }
	 preempt_enable()

	-ss