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

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Date:	Wed, 17 Oct 2007 20:27:37 +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:
> [Resending, directly to a couple likely suspects this time, in the
> hope of getting a reply... thanks]
> 
> I have a question about the synchronization of file_operations: is it
> intended that the .release method of a file can be called while a
> .read or .write method is still running for that file?

> I've written a quick test case that seems to show this happen in
> practice, and reading the code in fs/open.c and fs/read_write.c I see
> no reason why this race can't happen: sys_read() and sys_write() just
> do fget_light(), which will not increment the file's reference count
> on the fast path, so a racing sys_close() from another thread could do
> the final fput() that ends up calling the file's .release method
> before the read or write has finished.

If you _have_ another thread to call close() from, you won't hit that
fast path.  Case closed.
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