drm/mm: Allow drm_mm_initialized() to be used outside of the locks

Mark up the potential racy read in drm_mm_initialized(), as we want a
cheap and cheerful check:

[  121.098731] BUG: KCSAN: data-race in _i915_gem_object_create_stolen [i915] / rm_hole
[  121.098766]
[  121.098789] write (marked) to 0xffff8881f01ed330 of 8 bytes by task 3568 on cpu 3:
[  121.098831]  rm_hole+0x64/0x140
[  121.098860]  drm_mm_insert_node_in_range+0x3d3/0x6c0
[  121.099254]  i915_gem_stolen_insert_node_in_range+0x91/0xe0 [i915]
[  121.099646]  _i915_gem_object_create_stolen+0x9d/0x100 [i915]
[  121.100047]  i915_gem_object_create_region+0x7a/0xa0 [i915]
[  121.100451]  i915_gem_object_create_stolen+0x33/0x50 [i915]
[  121.100849]  intel_engine_create_ring+0x1af/0x280 [i915]
[  121.101242]  __execlists_context_alloc+0xce/0x3d0 [i915]
[  121.101635]  execlists_context_alloc+0x25/0x40 [i915]
[  121.102030]  intel_context_alloc_state+0xb6/0xf0 [i915]
[  121.102420]  __intel_context_do_pin+0x1ff/0x220 [i915]
[  121.102815]  i915_gem_do_execbuffer+0x46b4/0x4c20 [i915]
[  121.103211]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[  121.103244]  drm_ioctl_kernel+0xe4/0x120
[  121.103269]  drm_ioctl+0x297/0x4c7
[  121.103296]  ksys_ioctl+0x89/0xb0
[  121.103321]  __x64_sys_ioctl+0x42/0x60
[  121.103349]  do_syscall_64+0x6e/0x2c0
[  121.103377]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  121.103403]
[  121.103426] read to 0xffff8881f01ed330 of 8 bytes by task 3109 on cpu 1:
[  121.103819]  _i915_gem_object_create_stolen+0x30/0x100 [i915]
[  121.104228]  i915_gem_object_create_region+0x7a/0xa0 [i915]
[  121.104631]  i915_gem_object_create_stolen+0x33/0x50 [i915]
[  121.105025]  intel_engine_create_ring+0x1af/0x280 [i915]
[  121.105420]  __execlists_context_alloc+0xce/0x3d0 [i915]
[  121.105818]  execlists_context_alloc+0x25/0x40 [i915]
[  121.106202]  intel_context_alloc_state+0xb6/0xf0 [i915]
[  121.106595]  __intel_context_do_pin+0x1ff/0x220 [i915]
[  121.106985]  i915_gem_do_execbuffer+0x46b4/0x4c20 [i915]
[  121.107375]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
[  121.107409]  drm_ioctl_kernel+0xe4/0x120
[  121.107437]  drm_ioctl+0x297/0x4c7
[  121.107464]  ksys_ioctl+0x89/0xb0
[  121.107489]  __x64_sys_ioctl+0x42/0x60
[  121.107511]  do_syscall_64+0x6e/0x2c0
[  121.107535]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200309121529.16497-1-chris@chris-wilson.co.uk

include/drm/drm_mm.h

@@ -272,7 +272,7 @@ static inline bool drm_mm_node_allocated(const struct drm_mm_node *node)
  */
 static inline bool drm_mm_initialized(const struct drm_mm *mm)
 {
-	return mm->hole_stack.next;
+	return READ_ONCE(mm->hole_stack.next);
 }
 
 /**