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()..

Thanks,
Fengguang