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

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Date:	Wed, 19 Sep 2007 14:07:49 -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 Tue, Sep 18, 2007 at 10:36:32AM +0400, Pavel Emelyanov wrote:
> J. Bruce Fields wrote:
> > I would also prefer a locking scheme that didn't rely on the BKL.  That
> > said, except for this race:
> 
> I would as well :) But I don't know the locking code good enough to
> start fixing. Besides, even if I send a patch series that handles this,
> I don't think that anyone will accept it, due to "this changes too much
> code", "can you prove you fixed all the places" and so on...

Several people have expressed interest in a locking scheme for locks.c
(and probably lockd) that doesn't depend on BKL, so I don't think it
would be ignored.  But, yes, it would have to be done very carefully;
there have been at least one or two previous attempts that failed.

> >>> (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?
> 
> Well, there's a long standing BUG in leases code - when we made all the
> checks in inserting lease, we call the locks_alloc_lock() and may fall
> asleep. Bu after the wakeup nobody re-checks for the things to change.

Ouch, yes, you're right.

> I suspect there are other bad places.

OK.  Thanks in advance for finding any!

--b.
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