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

[Kristian Høgsberg]

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@xxxxxxxxxxxxx wrote:
> > > From: Kristian HÃgsberg <krh@xxxxxxxxxx>
> > > 
> > > 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 HÃgsberg <krh@xxxxxxxxxx>
> > > ---
> > > 
> > > 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


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