drm/i915/gem: Avoid synchronous binds deep within locks

On bxt, we require a VT'd w/a to serialise all GGTT updates with memory
transfers, and use stop_machine() for this purpose. stop_machine() is a
global serialisation barrier and so dangerous to use from within
critical sections, as the stop_machine() will wait for all cpus to enter
the stop_machine callback, and those cpus may be waiting for the
critical section already held.

Fixes: d7085b0f ("drm/i915/gem: Poison stolen pages before use")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201027184759.29888-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_stolen.c

@@ -505,6 +505,9 @@ static void dbg_poison(struct i915_ggtt *ggtt,
 	if (!drm_mm_node_allocated(&ggtt->error_capture))
 		return;
 
+	if (ggtt->vm.bind_async_flags & I915_VMA_GLOBAL_BIND)
+		return; /* beware stop_machine() inversion */
+
 	GEM_BUG_ON(!IS_ALIGNED(size, PAGE_SIZE));
 
 	mutex_lock(&ggtt->error_mutex);