linux-kernel - Re: [PATCH] Wake up mandatory locks waiter on chmod

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Message-ID: <20070917145934.GA4957@fieldses.org>
Date:	Mon, 17 Sep 2007 10:59:34 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Pavel Emelyanov <xemul@...nvz.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org
Subject: Re: [PATCH] Wake up mandatory locks waiter on chmod

On Mon, Sep 17, 2007 at 10:37:56AM +0400, Pavel Emelyanov wrote:
> J. Bruce Fields wrote:
> > 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:

OK, but why not just remove the inode->i_flock check there?  I can't see
how it helps anyway.

> > 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...

I would also prefer a locking scheme that didn't rely on the BKL.  That
said, except for this race:

> > (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.)

... I'm not aware of other races in the existing file-locking code.  It
sounds like you might be.  Could you give specific examples?

--b.
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