We cannot nest i915_reset_trylock() as the inner may wait for the
I915_RESET_BACKOFF which in turn is waiting upon sync_srcu who is
waiting for our outermost lock. As we take the reset srcu around the
fence update, we have to defer taking it in i915_gem_fault() until after
we acquire the pin on the fence to avoid nesting. This is a little ugly,
but still works. If a reset occurs between i915_vma_pin_fence() and the
second reset lock, the reset will restore the fence register back to the
pinned value before the reset lock allows us to proceed (our mmap won't
be revoked as we haven't yet marked it as being a userfault as that
requires us to hold the reset lock), so the pagefault is still
serialised with the revocation in reset.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109605
Fixes: 2caffbf1 ("drm/i915: Revoke mmaps and prevent access to fence registers across reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190212130831.14425-1-chris@chris-wilson.co.uk