linux-kernel - Re: [PATCH 04/23] vfs: Introduce infrastructure for revoking a file

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Date:	Tue, 2 Jun 2009 09:14:11 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>,
	Tejun Heo <tj@...nel.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 04/23] vfs: Introduce infrastructure for revoking a file

On Mon, Jun 01, 2009 at 02:50:29PM -0700, Eric W. Biederman wrote:
> From: Eric W. Biederman <ebiederm@...ssion.com>
> 
> Introduce the file_hotplug_lock to protect file->f_op, file->private,
> file->f_path from revoke operations.
> 
> The file_hotplug_lock is used typically as:
> error = -EIO;
> if (!file_hotplug_read_trylock(file))
> 	goto out;
> ....
> file_hotplug_read_unlock(file);

Why is it called hotplug? Does it have anything to do with hardware?
Because every concurrently changed software data structure in the
kernel can be "hot"-modified, right?

Wouldn't file_revoke_lock be more appropriate?


> In addition for a complete solution we need:
> - A reliable way the file structures that we need to revoke.
> - To wait for but not tamper with ongoing file creation and cleanup.
> - A guarantee that all with user space controlled duration are removed.
> 
> The file_hotplug_lock has a very unique implementation necessitated by
> the need to have no performance impact on existing code.  Classic locking

Well, it isn't no performance impact. Function calls, branches, icache
and dcache...

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