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Message-ID: <46EE20C4.1060405@openvz.org>
Date: Mon, 17 Sep 2007 10:37:56 +0400
From: Pavel Emelyanov <xemul@...nvz.org>
To: "J. Bruce Fields" <bfields@...ldses.org>
CC: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
devel@...nvz.org
Subject: Re: [PATCH] Wake up mandatory locks waiter on chmod
J. Bruce Fields wrote:
> On Thu, Sep 13, 2007 at 06:30:43PM +0400, Pavel Emelyanov wrote:
>> When the process is blocked on mandatory lock and someone changes
>> the inode's permissions, so that the lock is no longer mandatory,
>> nobody wakes up the blocked process, but probably should.
>
> I suppose so. Does anyone actually use mandatory locking?
:) Good question.
> Would it be worth adding a
>
> if (MANDATORY_LOCK(inode))
> return;
>
> to the beginning of locks_wakeup_mandatory() to avoid walking the list
> of locks in that case? Perhaps setattr is rare enough that this just
> isn't worth caring about.
>
> Is there a small chance that a lock may be applied after this check:
>
>> + mandatory = (inode->i_flock && MANDATORY_LOCK(inode));
>> +
>
> but early enough that someone can still block on the lock while the file
> is still marked for mandatory locking? (And is the inode->i_flock check
> there really necessary?)
There is, but as you have noticed:
> Well, there are probably worse races in the mandatory locking code.
...there are. The inode->i_lock is protected with lock_kernel() only
and is not in sync with any other checks for inodes. This is sad :(
but a good locking for locks is to be done...
> (For example, my impression is that a mandatory lock can be applied just
> after the locks_mandatory_area() checks but before the io actually
> completes.)
>
> --b.
Thanks,
Pavel
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