Re: Kernel boot hangs on commit "switch fput to task_work_add"

[Fengguang Wu <wfg@linux.intel.com>]

On Sat, Jul 14, 2012 at 02:05:10PM +0100, Al Viro wrote:
> On Tue, Jul 10, 2012 at 10:18:30PM +0800, wfg@linux.intel.com wrote:
> > Hi Al,
> > 
> > The linux-next kernel reliably hung after this line:
> > 
> > [    4.846260] debug: unmapping init [mem 0xffff88000182a000-0xffff8800019fffff]
> > 
> > And it's bisected to commit:
> > 
> >         commit 4a9ffe81385c2af04f296bea05482f34e02ea10d
> >         Author: Al Viro <viro@zeniv.linux.org.uk>
> >         Date:   Sun Jun 24 09:56:45 2012 +0400
> > 
> >             switch fput to task_work_add
> > 
> >             ... and schedule_work() for interrupt/kernel_thread callers
> >             (and yes, now it *is* OK to call from interrupt).
> > 
> > I tried add this debug aid:
> > 
> > init_post(void):
> > +       printk(KERN_WARNING "flush_delayed_fput\n");
> >         flush_delayed_fput();
> > +       printk(KERN_WARNING "flush_delayed_fput done\n");
> > 
> > And then it hangs after "flush_delayed_fput done". So it's not directly
> > freezing inside flush_delayed_fput()..
> 
> Could you post a stack trace, etc.?  I'll try to reproduce that one, obviously,
> but...

Yeah, I'm wondering (and will try) if it will help sending SYSRQ into
kvm guest. It's rather clueless without a back trace..

Thanks,
Fengguang