Re: [RESEND] file operations: release can race with read/write?

[Al Viro <viro@ftp.linux.org.uk>]

On Wed, Oct 17, 2007 at 11:30:44AM -0700, Roland Dreier wrote:
> static unsigned long flag;
> 
> static int cw_open(struct inode *inode, struct file *filp)
> {
> 	return 0;
> }
> 
> static int cw_close(struct inode *inode, struct file *filp)
> {
> 	clear_bit(1, &flag);
> 
> 	msleep(1);
> 
> 	if (test_and_set_bit(1, &flag))
> 		printk(KERN_ERR "Write raced with close?\n");

Of _course_ you can trigger that - just open two files and have write
on one of them while another gets closed.

That is certainly possible, but it has nothing whatsoever with the
race you've described - you have two different objects and method
call on one of them might happen when destructor is called on another.

You have protection against ->release() for a struct file called
while you are in ->write() for _same_ struct file.