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

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Date:	Mon, 21 Mar 2016 15:32:12 +0100
From:	Jan Kara <jack@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Byungchul Park <byungchul.park@....com>,
	Sergey Senozhatsky <sergey.senozhatsky@...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, Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH v5 1/2] printk: Make printk() completely async

On Mon 21-03-16 18:28:48, Sergey Senozhatsky wrote:
> On (03/21/16 17:47), Byungchul Park wrote:
> [..]
> > > Is there any reason why you don't put the wake_up_process() out of the
> > > critical section with my suggestion, even though it can solve the infinite
> > > recuresion you worried about?
> > 
> > Just to be sure, whether you take my suggestion or not is not important.
> > I just suggested it in this thread since it can solve what you worried
> > about. That's all. I can post it in another thread though. Why don't you
> > consider it so that yours don't miss any printk message? Do you think there
> > are any problems in my suggestion?
> 
> we have 2 spin locks in vprintk_emit() -- logbuf_lock and sem->lock. and N
> CPUs can concurrently lockup on those two locks, which already makes a
> single static pointer in spiun_dump() questionable.
> 
> logbug_lock *theoretically* can detect and handle recursive printk()s,
> there is no way to catch sem->lock spin_dump() at the moment (but that's
> not the point).
> 
> there are 2 new spin locks in vprintk_emit() -- p->pi_lock and rq->lock.

Actually, this is not true. These locks are already in vprintk_emit() via
the up(&console_sem) call from console_unlock() since up() can call
wake_up() which needs the same locks as wake_up_process().

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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR