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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-02-19 at 10:19 +0100, Peter Zijlstra wrote:
> On Wed, 2009-02-18 at 11:38 -0500, krh@bitplanet.net wrote:
> > From: Kristian Høgsberg <krh@redhat.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@redhat.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 :/

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.

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.