RE: Isn't there race issue during fput() and the dentry_open()?

["Tomita, Haruo" <haruo.tomita@toshiba.co.jp>]

Hi Viro,

Thank you for your replay.

> > Stack traceback for pid 2130
> > 0xf717f1b0	2130	1	1	0	R	
> 0xf717f400 *irqbalance
> > ESP	EIP	Function (args)
> > 0xf75bfe38 0xc02d04b2 _spin_lock+0x2e (0xf7441a80)
> > 0xf75bff34 0xc015667c file_move+0x14 (0xf63080e4, 
> 0xf75bff58, 0x0, 0xf74bf000)
> > 0xf75bff40 0xc0154e37 dentry_open+0xb9 (0xf63080e4, 
> 0xf7f5ad80, 0xc02d00e6, 0x100100, 0x246)
> > 0xf75bff58 0xc0154d78 filp_open+0x36
> > 0xf75bffb4 0xc0155079 sys_open+0x31
> > 0xf75bffc4 0xc02d196f syscall_call+0x7
> > 
> > The patch was made. Is this patch right?
> > 
> > diff -urN linux-2.6.12-rc1.orig/fs/file_table.c 
> linux-2.6.12-rc1/fs/file_table.c
> > --- linux-2.6.12-rc1.orig/fs/file_table.c	2005-03-02 
> 16:37:47.000000000 +0900
> > +++ linux-2.6.12-rc1/fs/file_table.c	2005-03-31 
> 17:50:46.323999320 +0900
> > @@ -209,11 +209,11 @@
> >  
> >  void file_kill(struct file *file)
> >  {
> > +	file_list_lock();
> >  	if (!list_empty(&file->f_list)) {
> > -		file_list_lock();
> >  		list_del_init(&file->f_list);
> > -		file_list_unlock();
> >  	}
> > +	file_list_unlock();
> >  }
> 
> This is absolutely useless.  What are you trying to protect 
> and how the
> hell could keeping a lock around that check prevent any sort 
> of deadlock?

I think that it is true not to be able to acquire file_list_lock()
that is called from file_move().
 
> Besides, who could possibly call fput() on struct file allocated by
> dentry_open() and do that before the latter returns a reference to
> that struct file?

Indeed, Is there a good method of debugging this issue?
In the check on the source, a doubtful place was not found except file_kill(). 
I might call not race of fput() and the dentry_open() but
the deadlock of file_list_lock(). 
--
Haruo
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