Merge branch 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

torvalds · torvalds · commit bd3fd451ff3b · 2016-08-18T13:45:48.000-07:00
Pull locking fixes from Ingo Molnar:
 "Two lockless_dereference() related fixes"

* 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/barriers: Suppress sparse warnings in lockless_dereference()
  Revert "drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference"
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
@@ -464,7 +464,7 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
 
 	/* Sometimes user space wants everything disabled, so don't steal the
 	 * display if there's a master. */
-	if (lockless_dereference(dev->master))
+	if (READ_ONCE(dev->master))
 		return false;
 
 	drm_for_each_crtc(crtc, dev) {
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
@@ -527,13 +527,13 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * object's lifetime is managed by something other than RCU.  That
  * "something other" might be reference counting or simple immortality.
  *
- * The seemingly unused void * variable is to validate @p is indeed a pointer
- * type. All pointer types silently cast to void *.
+ * The seemingly unused size_t variable is to validate @p is indeed a pointer
+ * type by making sure it can be dereferenced.
  */
 #define lockless_dereference(p) \
 ({ \
 	typeof(p) _________p1 = READ_ONCE(p); \
-	__maybe_unused const void * const _________p2 = _________p1; \
+	size_t __maybe_unused __size_of_ptr = sizeof(*(p)); \
 	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
 	(_________p1); \
 })
