linux-kernel - Re: [RFC][PATCH 5/9] vfs: Introduce basic infrastructure for revoking a file

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Message-ID: <20090414161240.73fe6bcd@bike.lwn.net>
Date:	Tue, 14 Apr 2009 16:12:40 -0600
From:	Jonathan Corbet <corbet@....net>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	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>
Subject: Re: [RFC][PATCH 5/9] vfs: Introduce basic infrastructure for
 revoking a file

Hi, Eric,

One little thing I noticed as I was looking at this...

> +int fops_substitute(struct file *file, const struct file_operations *f_op,
> +			struct vm_operations_struct *vm_ops)
> +{

 [...]

> +	/*
> +	 * Wait until there are no more callers in the original
> +	 * file_operations methods.
> +	 */
> +	while (atomic_long_read(&file->f_use) > 0)
> +		schedule_timeout_interruptible(1);

You use an interruptible sleep here, but there's no signal check to get you
out of the loop.  So it's not really interruptible.  If f_use never goes to
zero (a distressingly likely possibility, I fear), this code will create
the equivalent of an unkillable D-wait state without ever actually showing
up that way in "ps".

Actually, now that I look, once you've got a signal pending you'll stay
in TASK_RUNNING, so the above could turn into a busy-wait.

Unless I've missed something...?

I have no idea what the right thing to do in the face of a signal would
be.  Perhaps the wait-for-zero and release() call stuff should be dumped
into a workqueue and done asynchronously?  OTOH, I can see a need to know
when the revoke operation is really done...

jon
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