linux-kernel - Re: [RFC][PATCH v5 1/2] printk: Make printk() completely async

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Date:	Tue, 22 Mar 2016 11:18:24 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Jan Kara <jack@...e.cz>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Petr Mladek <pmladek@...e.com>,
	Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH v5 1/2] printk: Make printk() completely async

On Tue, Mar 22, 2016 at 02:11:05AM +0900, Sergey Senozhatsky wrote:
> On (03/21/16 16:33), Jan Kara wrote:
> [..]
> > > > And by calling wake_up_process() under logbuf_lock, you actually introduce
> > > > recursion issues for printk_deferred() messages which are supposed to be
> > > > working from under rq->lock and similar. So I think you have to keep this
> > > > section outside of logbuf_lock.
> > > 
> > > hm, in_sched (printk_deferred()) messages are printed by
> > > irq work->wake_up_klogd_work_func(), not by wake_up_process()
> > > from vprintk_emit(). or am I missing something?
> > 
> > Think of following:
> > 
> > some function
> >   printk()
> >     vprintk_emit()
> >       spin_lock(&logbuf_lock);
> >       ...
> >       wake_up_process()
> >         printk_deferred()
> >           vprintk_emit() -> recursion on logbuf_lock
> 
> uh, indeed. I was more concerned about printk() calls that are
> troublemakers and are already in wake_up_process() - spin_dump()s.
> but yes, braking printk_deferred() in this case is a regression.

Already said any kind of printk() cannot work within logbuf_lock. :-(

> thanks for pointing that out. and also thanks to Byungchul.

My pleasure.