[Intel-gfx] [PATCH 4/4] drm/i915: Support for pread/pwrite from/to non shmem backed objects

[Daniel Vetter]

On Wed, Jul 22, 2015 at 04:46:00PM +0100, Tvrtko Ursulin wrote:
> But also, why it is even necessary to drop the mutex here?

Locking inversion with our own pagefault handler. Happens since at least
mesa can return gtt mmaps to gl clients, which can then in turn try to
upload that with pwrite somewhere else. Also userspace could be just plain
nasty and try to deadlock the kernel ;-)

Dropping the struct_mutex is SOP for the copy_from/to_user slowpath.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch