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

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Message-ID: <20160321145832.GA545@swordfish>
Date:	Mon, 21 Mar 2016 23:58:32 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	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
Subject: Re: [RFC][PATCH v5 1/2] printk: Make printk() completely async

Hello Jan,

On (03/21/16 15:32), Jan Kara wrote:
[..]
> > 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().

true. I meant new locks (which come with printk_kthread). the already
existing locks and problems were not addressed.

> 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?

I'll take a look and re-spin.

	-ss