linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Date:   Thu, 29 Jun 2017 20:24:03 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     sergey.senozhatsky.work@...il.com, pmladek@...e.com
Cc:     sergey.senozhatsky@...il.com, rostedt@...dmis.org, jack@...e.cz,
        akpm@...ux-foundation.org, peterz@...radead.org, rjw@...ysocki.net,
        ebiederm@...ssion.com, gregkh@...uxfoundation.org, jslaby@...e.com,
        pavel@....cz, andi@...as.de, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

Sergey Senozhatsky wrote:
> On (06/28/17 14:19), Petr Mladek wrote:
> > Our two proposals are very close after all. I suggest to make
> > the following changes in your patch:
> > 
> >   + Remove the waiting for another console_lock owner. It is
> >     too tricky.
> 
> we lose the printing guarantees this way. what if printk_kthread
> doesn't wake up after all? the whole point of this design twist
> (and previous discussions) was that people spoke up and said that
> they want printk to do the thing it was doing for decades. even if
> it would cause lockup reports sometimes (but it doesn't seem to be
> such a common problem after all. how many people see printk lockup
> reports more or less regularly?).

I do see printk lockup regularly because warn_alloc() from __alloc_pages_may_oom()
is basically doing

   while (1) {
     dump_stack();
     cond_resched();
   }

as long as somebody is looping inside console_unlock() with oom_lock mutex held.
Since warn_alloc() is currently a local DoS attack vector, I'm waiting for this
printk offloading patch to be merged into mainline, but it seems that this
offloading might not be able to become a solution to warn_alloc() lockup problem.