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Date: Tue, 2 Jun 2009 08:16:44 +0300
Message-ID: <84144f020906012216n715a04d0ha492abc12175816@mail.gmail.com>
Subject: Re: [PATCH 04/23] vfs: Introduce infrastructure for revoking a file
From: Pekka Enberg <penberg@cs.helsinki.fi>
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Sender: owner-linux-mm@kvack.org
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Hugh Dickins <hugh@veritas.com>, Tejun Heo <tj@kernel.org>, Alexey Dobriyan <adobriyan@gmail.com>, Linus Torvalds <torvalds@linux-foundation.org>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Greg Kroah-Hartman <gregkh@suse.de>, Nick Piggin <npiggin@suse.de>, Andrew Morton <akpm@linux-foundation.org>, Christoph Hellwig <hch@infradead.org>, "Eric W. Biederman" <ebiederm@aristanetworks.com>
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Hi Eric,

On Tue, Jun 2, 2009 at 12:50 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> +#ifdef CONFIG_FILE_HOTPLUG
> +
> +static bool file_in_use(struct file *file)
> +{
> + =A0 =A0 =A0 struct task_struct *leader, *task;
> + =A0 =A0 =A0 bool in_use =3D false;
> + =A0 =A0 =A0 int i;
> +
> + =A0 =A0 =A0 rcu_read_lock();
> + =A0 =A0 =A0 do_each_thread(leader, task) {
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 for (i =3D 0; i < MAX_FILE_HOTPLUG_LOCK_DEP=
TH; i++) {
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if (task->file_hotplug_lock=
[i] =3D=3D file) {
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 in_use =3D =
true;
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 goto found;
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 }
> + =A0 =A0 =A0 } while_each_thread(leader, task);
> +found:
> + =A0 =A0 =A0 rcu_read_unlock();
> + =A0 =A0 =A0 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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