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Date: Mon, 21 Mar 2016 23:58:32 +0900
From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Byungchul Park <byungchul.park@lge.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.com>,
        Petr Mladek <pmladek@suse.com>, Tejun Heo <tj@kernel.org>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH v5 1/2] printk: Make printk() completely async
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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