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

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Message-ID: <20071017193320.GK8181@ftp.linux.org.uk>
Date:	Wed, 17 Oct 2007 20:33:21 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RESEND] file operations: release can race with read/write?

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.
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