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Subject[PATCH v2 4/5] locking/barriers: Kill lockless_dereference()
Date
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@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
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 배리어들은 SMP 시스템에서도 UP 시스템에서도 SMP 효
참고하세요.


- (*) lockless_dereference();
-
- 이 함수는 smp_read_barrier_depends() 데이터 의존성 배리어를 사용하는
- 포인터 읽어오기 래퍼(wrapper) 함수로 생각될 수 있습니다.
-
- 객체의 라이프타임이 RCU 외의 메커니즘으로 관리된다는 점을 제외하면
- rcu_dereference() 와도 유사한데, 예를 들면 객체가 시스템이 꺼질 때에만
- 제거되는 경우 등입니다. 또한, lockless_dereference() 은 RCU 와 함께
- 사용될수도, RCU 없이 사용될 수도 있는 일부 데이터 구조에 사용되고
- 있습니다.
-
-
(*) 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
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