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Date: Wed, 16 Mar 2016 16:56:05 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
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        Tejun Heo <tj@kernel.org>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async
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On (03/16/16 16:30), Byungchul Park wrote:
>
> Do you mean the wake_up_process() in console_unlock?

no, I meant wake_up_process(printk_kthread), the newly added one.


-- if we are going to have wake_up_process() in wake_up_klogd_work_func(),
then we need `in_sched' message to potentially trigger a recursion chain

wake_up_klogd_work_func()->wake_up_process()->printk()->wake_up_process()->printk()...

to break this printk()->wake_up_process()->printk(), we need wake_up_process() to
be under the logbuf lock; so vprintk_emit()'s if (logbuf_cpu == this_cpu) will act.


-- if we are going to have wake_up_process() in console_unlock(), then

console_unlock()->{up(), wake_up_process()}->printk()->{console_lock(), console_unlock()}->{up(), wake_up_process()}->printk()...

is undetectable... by the time console_unlock() calls wake_up_process() there
are no printk() locks that this CPU owns.


> I said they should be kept *out of* the critical section. :-)
> Otherwise, it can recurse us forever.

can you explain?

	-ss