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

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Date:	Tue, 2 Jun 2009 08:16:44 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
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>,
	Nick Piggin <npiggin@...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

Hi Eric,

On Tue, Jun 2, 2009 at 12:50 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> +#ifdef CONFIG_FILE_HOTPLUG
> +
> +static bool file_in_use(struct file *file)
> +{
> +       struct task_struct *leader, *task;
> +       bool in_use = false;
> +       int i;
> +
> +       rcu_read_lock();
> +       do_each_thread(leader, task) {
> +               for (i = 0; i < MAX_FILE_HOTPLUG_LOCK_DEPTH; i++) {
> +                       if (task->file_hotplug_lock[i] == file) {
> +                               in_use = true;
> +                               goto found;
> +                       }
> +               }
> +       } while_each_thread(leader, task);
> +found:
> +       rcu_read_unlock();
> +       return in_use;
> +}

This seems rather heavy-weight. If we're going to use this
infrastructure for forced unmount, I think this will be a problem.

Can't we two this in two stages: (1) mark a bit that forces
file_hotplug_read_trylock to always fail and (2) block until the last
remaining in-kernel file_hotplug_read_unlock() has executed?

                        Pekka
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