linux-kernel - Re: [GIT PULL] printk for 4.13

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Date:   Thu, 6 Jul 2017 09:53:28 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] printk for 4.13

On (07/06/17 09:44), Sergey Senozhatsky wrote:
> > No, this is entirely buggered.
> > 
> > You can't just do
> > 
> >    raw_spin_is_locked(&logbuf_lock)
> > 
> > to test whether you can get the logbuf_lock and then call vprintk_default().
> > 
> > It's not just about deadlock avoidance, the code will call things like
> > down_trylock() on the console semaphore, and that operation is
> > fundamentally not NMI-safe. The semaphore count is literally protected
> > by a irq-safe (BUT NOT NMI-SAFE!) semaphore spinlock.
> >
> > So now you can instead deadlock just on the internal console semaphore
> > spinlock (ie somebody is doing "console_lock()", NMI comes in, *BOOM*.
> 
> you are right. but
> 

just in case if my previous email was too hard to follow, may be the
link below will explain it in a less broken language. (I'm sorry, I'm
on medication now and didn't have enough coffee yet.)

https://marc.info/?l=linux-kernel&m=149265908228203&w=2

	-ss