linux-kernel - Re: lockdep splat from ioctl and mmap fops sharing lock

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Message-ID: <8763njjmqt.fsf@saeurebad.de>
Date:	Thu, 23 Oct 2008 18:32:26 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrea Righi <righi.andrea@...il.com>,
	Dave Jones <davej@...hat.com>, Dave Airlie <airlied@...hat.com>
Subject: Re: lockdep splat from ioctl and mmap fops sharing lock

Stefan Richter <stefanr@...6.in-berlin.de> writes:

> Johannes Weiner wrote:
>> raw1394_ioctl() doing usercopy under fi->state_mutex
>> raw1394_mmap() taking fi->state_mutex under mmap_sem
>
> The state_mutex in raw1394 however was introduced by me in patches
> written against 2.6.26 and 2.6.27-rcs.  And I tested the ioctls and I'd
> like to think that I also tested mmaps.  But maybe I didn't.  Of course
> I ahve all sorts of lockdep options enabled.
>
> So, was the usage of mmap_sem changed after 2.6.27 or were my tests
> insufficient?

In linux-next/-mm, copy_to/from_user have lockdep annotations telling
that they might fault and therefor acquire the mmap_sem in #PF.

But since faults on ioctl parameters are so rare, without these
annotations you would probably never see a warning.

        Hannes
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