Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async

From: Byungchul Park
Date: Wed Mar 16 2016 - 06:34:47 EST

On Wed, Mar 16, 2016 at 04:56:05PM +0900, Sergey Senozhatsky wrote:
> 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.

I got it. You are talking about wake_up_process() in Petr's patch.

> -- 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.

I am curious about how you make the wake_up_process() call and I may want
to talk about it at the next spin. Anyway, then we will lose the last
message when "if (logbuf_cpu == this_cpu)" acts. Is it acceptible?

IMHO it's not a good choice to use wake_up() and friend within a printk()
since it can additionally cause another recursion. Of course, it does not
happen if the condition (logbuf_cpu == this_cpu) acts. But I don't think
it's good to rely on the condition with losing a message. Anyway I really
really want to see your next spin and talk.

> -- 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()...

This cannot happen. console_lock() cannot continue because the prior
console_unlock() does not release console_sem.lock yet when
wake_up_process() is called. Only a deadlock exists. And my patch solves
the problem so that the deadlock cannot happen.

My patch is as you already know.

> 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?

Sorry. I was confused. I was wrong.

> -ss