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

[Yong Zhang <yong.zhang0@gmail.com>]

On Fri, Nov 18, 2011 at 11:01:00AM -0800, Andrew Morton wrote:
> 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().

Hmm, after revisting the code, seems console_unlock() can't be
preempted when helding console_sem. IOW, stop_machine() can't
run if someone loops in printing something.

So the problem only affect -rt, sorry for the noise.

Thanks,
Yong

> 
> 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...