drm/i915: Don't call synchronize_rcu_expedited under struct_mutex
Only call synchronize_rcu_expedited after unlocking struct_mutex to
avoid deadlock because the workqueues depend on struct_mutex.
>From original patch by Andrea:
synchronize_rcu/synchronize_sched/synchronize_rcu_expedited() will
hang until its own workqueues are run. The i915 gem workqueues will
wait on the struct_mutex to be released. So we cannot wait for a
quiescent state using those rcu primitives while holding the
struct_mutex or it creates a circular lock dependency resulting in
kernel hangs (which is reproducible but goes undetected by lockdep).
Fixes: 3d3d18f086cd ("drm/i915: Avoid rcu_barrier() from reclaim paths (shrinker)") Reported-by: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Chris Wilson <chris@chris-wilson.co.uk> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Jani Nikula <jani.nikula@intel.com> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>