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

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Date:   Tue, 16 Jan 2018 10:08:44 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...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 Tue 2018-01-16 14:16:22, Sergey Senozhatsky wrote:
> 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()

Great catch! And good to know about it when designing further
improvements.

Best Regards,
Petr