Re: [RFC] [PATCH] printk: Don't emit console_cpu_notify() for CPU_DYING

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 18 Nov 2011 10:19:24 +0800
Yong Zhang <yong.zhang0@gmail.com> wrote:

> When running -rt, I got some warning when doing cpu hotplug,
> for details please check:
> http://marc.info/?l=linux-kernel&m=132143661616297&w=2
> 
> But seems the problem also exists in mainline, a typical
> scenario is:
> 
>         CPU A                           CPU B
> 
> doing printk with console_sem held
>                                 take_cpu_down();
>                                   cpu_notify(CPU_DYING);
>                                     console_cpu_notify();
>                                       console_lock();
>                                         down(&console_sem);
>                                           *BANG*
>   up(&console_sem);

This doesn't describe a deadlock.  For a deadlock to occur, something
must prevent CPU A from proceeding to the up().

Are you able to more fully describe this?  I'm suspecting that CPU A
got shut down by CPU B?  If so, shutting down a CPU while it holds
locks seems rather bad.  Or perhaps it's something else...