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

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Date:	Tue, 2 Jun 2009 10:08:14 +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 9:51 AM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Pekka Enberg <penberg@...helsinki.fi> writes:
>
>> 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?
>
> Yes there is room for more optimization in the slow path.
> I haven't noticed being a problem yet so I figured I would start
> with stupid and simple.

Yup, just wanted to point it out.

On Tue, Jun 2, 2009 at 9:51 AM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> I can easily see two passes.  The first setting the flag an calling
> f_op->dead.  The second some kind of consolidate walk through the task
> list, allowing checking on multiple files at once.
>
> I'm not ready to consider anything that will add cost to the fast
> path in the file descriptors though.

Makes sense.

                        Pekka
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