linux-kernel - [tip:locking/core] locking/barriers: Kill lockless

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Date:   Tue, 24 Oct 2017 05:47:25 -0700
From:   tip-bot for Will Deacon <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, torvalds@...ux-foundation.org, mingo@...nel.org,
        peterz@...radead.org, tglx@...utronix.de,
        paulmck@...ux.vnet.ibm.com, will.deacon@....com,
        linux-kernel@...r.kernel.org
Subject: [tip:locking/core] locking/barriers: Kill lockless_dereference()

Commit-ID:  59ecbbe7b31cd2d86ff9a9f461a00f7e7533aedc
Gitweb:     https://git.kernel.org/tip/59ecbbe7b31cd2d86ff9a9f461a00f7e7533aedc
Author:     Will Deacon <will.deacon@....com>
AuthorDate: Tue, 24 Oct 2017 11:22:49 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 24 Oct 2017 13:17:33 +0200

locking/barriers: Kill lockless_dereference()

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.

Signed-off-by: Will Deacon <will.deacon@....com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1508840570-22169-5-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 b759a60..470a682 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 a7a8132..ec3b46e 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 7d7b77d..5a1cab4 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 */