Re: [PATCH] drm: Fix lock order reversal between mmap_sem and struct_mutex.

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2009-02-25 at 00:15 -0800, Eric Anholt wrote:
> > Or you could get_user_pages() to fault the user pages and pin them, and
> > then do pagefault_disable() and use copy_from_user_inatomic or such, and
> > release the pages again.
> 
> I started poking at this today, since the get_user_pages sounded like
> the solution.  Only then I noticed: when we unbind an existing object,
> we have to unmap_mapping_range to clear the clients' mappings to it in
> the GTT, which needs to happen while the struct lock (protecting the gtt
> structure and the gtt to object mappings) is held.  So for fault we have
> mmap_sem held to struct mutex taken for poking at the gtt structure, and
> for unbind we have struct mutex held to mmap_sem taken to clear
> mappings.

So it again comes down to the fact that you cannot pin a gtt object
without also holding this struct_mutex?

Normally such things are done by elevating a refcount so that both
regular frees and reclaim gets delayed until you're done with the
object.