linux-kernel - Re: [PATCH] drm: Take mmap_sem up front to avoid lock order violations.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1235054980.31384.16.camel@gaara.bos.redhat.com>
Date:	Thu, 19 Feb 2009 09:49:40 -0500
From:	Kristian Høgsberg <krh@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	krh@...planet.net, eric@...olt.net,
	Wang Chen <wangchen@...fujitsu.com>, dri-devel@...ts.sf.net,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH] drm: Take mmap_sem up front to avoid lock order
 violations.

On Thu, 2009-02-19 at 11:33 +0100, Peter Zijlstra wrote:
> On Thu, 2009-02-19 at 10:19 +0100, Peter Zijlstra wrote:
> > On Wed, 2009-02-18 at 11:38 -0500, krh@...planet.net wrote:
> > > From: Kristian Høgsberg <krh@...hat.com>
> > > 
> > > A number of GEM operations (and legacy drm ones) want to copy data to
> > > or from userspace while holding the struct_mutex lock.  However, the
> > > fault handler calls us with the mmap_sem held and thus enforces the
> > > opposite locking order.  This patch downs the mmap_sem up front for
> > > those operations that access userspace data under the struct_mutex
> > > lock to ensure the locking order is consistent.
> > > 
> > > Signed-off-by: Kristian Høgsberg <krh@...hat.com>
> > > ---
> > > 
> > > Here's a different and simpler attempt to fix the locking order
> > > problem.  We can just down_read() the mmap_sem pre-emptively up-front,
> > > and the locking order is respected.  It's simpler than the
> > > mutex_trylock() game, avoids introducing a new mutex.
> > > 
> 
> OK let me try that again -- my initial response was a tad curt :/

No that's fair, I was aware that the patch was probably borderline and
got the feedback I was looking for ;)

> While I appreciate your efforts in fixing GEM (I too have an interest in
> seeing it done), I cannot support your patch.
> 
> Firstly, you're using mmap_sem well outside its problem domain, this is
> bad form. Furthermore, holding it for extended durations for no good
> reason affects all other users.

Yup, agree.

> Secondly, mmap_sem is not a recursive lock (very few kernel locks are,
> and we generally frown upon recursive locking schemes), this means that
> the fault handler still cannot function properly.

I understand, but we take it twice only as a read lock, so that should
work, right?  We prevent the deadlock the lockdep validator warned about
and as far as I can see, the patch doesn't introduce a new one.  But
other than that I agree with the frowning on recursive locking, it's too
often used to paper over badly thought out locking.

cheers,
Kristian


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/