[PATCH v2 4/5] locking/barriers: Kill lockless_dereference()

[Will Deacon]

lockless_dereference() is a nice idea, but it gained little traction in
kernel code since its introduction three years ago. This is partly
because it's a pain to type, but also because using READ_ONCE() instead
has worked correctly on all architectures apart from Alpha, which is a
fully supported but somewhat niche architecture these days.

Now that READ_ONCE() has been upgraded to contain an implicit
smp_read_barrier_depends() and the few callers of lockless_dereference()
have been converted, we can remove lockless_dereference() altogether.

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 Documentation/memory-barriers.txt                    | 12 ------------
 Documentation/translations/ko_KR/memory-barriers.txt | 12 ------------
 include/linux/compiler.h                             | 20 --------------------
 3 files changed, 44 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index b759a60624fd..470a682f3fa4 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1886,18 +1886,6 @@ There are some more advanced barrier functions:
      See Documentation/atomic_{t,bitops}.txt for more information.
 
 
- (*) lockless_dereference();
-
-     This can be thought of as a pointer-fetch wrapper around the
-     smp_read_barrier_depends() data-dependency barrier.
-
-     This is also similar to rcu_dereference(), but in cases where
-     object lifetime is handled by some mechanism other than RCU, for
-     example, when the objects removed only when the system goes down.
-     In addition, lockless_dereference() is used in some data structures
-     that can be used both with and without RCU.
-
-
  (*) dma_wmb();
  (*) dma_rmb();
 
diff --git a/Documentation/translations/ko_KR/memory-barriers.txt b/Documentation/translations/ko_KR/memory-barriers.txt
index a7a813258013..ec3b46e27b7a 100644
--- a/Documentation/translations/ko_KR/memory-barriers.txt
+++ b/Documentation/translations/ko_KR/memory-barriers.txt
@@ -1858,18 +1858,6 @@ Mandatory ëëìëì SMP ììíììë UP ììíììë SMP í
      ìêíìì.
 
 
- (*) lockless_dereference();
-
-     ì íìë smp_read_barrier_depends() ëìí ììì ëëìë ììíë
-     íìí ìììê ëí(wrapper) íìë ìêë ì ììëë.
-
-     êìì ëìííìì RCU ìì ëìëììë êëëëë ìì ììíë
-     rcu_dereference() ìë ììíë, ìë ëë êìê ììíì êì ëìë
-     ìêëë êì ëìëë.  ëí, lockless_dereference() ì RCU ì íê
-     ììëìë, RCU ìì ììë ìë ìë ìë ëìí êìì ììëê
-     ììëë.
-
-
  (*) dma_wmb();
  (*) dma_rmb();
 
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 7d7b77da9716..5a1cab48442c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -346,24 +346,4 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	(volatile typeof(x) *)&(x); })
 #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
 
-/**
- * lockless_dereference() - safely load a pointer for later dereference
- * @p: The pointer to load
- *
- * Similar to rcu_dereference(), but for situations where the pointed-to
- * object's lifetime is managed by something other than RCU.  That
- * "something other" might be reference counting or simple immortality.
- *
- * The seemingly unused variable ___typecheck_p validates that @p is
- * indeed a pointer type by using a pointer to typeof(*p) as the type.
- * Taking a pointer to typeof(*p) again is needed in case p is void *.
- */
-#define lockless_dereference(p) \
-({ \
-	typeof(p) _________p1 = READ_ONCE(p); \
-	typeof(*(p)) *___typecheck_p __maybe_unused; \
-	smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
-	(_________p1); \
-})
-
 #endif /* __LINUX_COMPILER_H */
-- 
2.1.4