linux-kernel - Re: Kernel boot hangs on commit "switch fput to task_work

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Message-ID: <20120714135849.GA1257@localhost>
Date:	Sat, 14 Jul 2012 21:58:49 +0800
From:	Fengguang Wu <wfg@...ux.intel.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Kernel boot hangs on commit "switch fput to task_work_add"

On Sat, Jul 14, 2012 at 09:48:49PM +0800, Fengguang Wu wrote:
> On Sat, Jul 14, 2012 at 02:05:10PM +0100, Al Viro wrote:
> > On Tue, Jul 10, 2012 at 10:18:30PM +0800, wfg@...ux.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@...iv.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...
> 
> Hi Al, here I got the output for
> 
> (qemu) sendkey alt-sysrq-l
> (qemu) sendkey alt-sysrq-t

I repeated that several times and here are the results.

(qemu) sendkey alt-sysrq-l
(qemu) sendkey alt-sysrq-l
(qemu) sendkey alt-sysrq-l
(qemu) sendkey alt-sysrq-t
(qemu) sendkey alt-sysrq-t
(qemu) quit

The user space shutdown is polling on something, which prevents the
system from reboot..

Thanks,
Fengguang

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