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@arm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1508840570-22169-5-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -1886,18 +1886,6 @@ There are some more advanced barrier functions:
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See Documentation/atomic_{t,bitops}.txt for more information.
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(*) lockless_dereference();
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This can be thought of as a pointer-fetch wrapper around the
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smp_read_barrier_depends() data-dependency barrier.
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This is also similar to rcu_dereference(), but in cases where
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object lifetime is handled by some mechanism other than RCU, for
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example, when the objects removed only when the system goes down.
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In addition, lockless_dereference() is used in some data structures
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that can be used both with and without RCU.
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(*) dma_wmb();
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(*) dma_rmb();
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@ -1858,18 +1858,6 @@ Mandatory 배리어들은 SMP 시스템에서도 UP 시스템에서도 SMP 효
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참고하세요.
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(*) lockless_dereference();
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이 함수는 smp_read_barrier_depends() 데이터 의존성 배리어를 사용하는
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포인터 읽어오기 래퍼(wrapper) 함수로 생각될 수 있습니다.
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객체의 라이프타임이 RCU 외의 메커니즘으로 관리된다는 점을 제외하면
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rcu_dereference() 와도 유사한데, 예를 들면 객체가 시스템이 꺼질 때에만
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제거되는 경우 등입니다. 또한, lockless_dereference() 은 RCU 와 함께
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사용될수도, RCU 없이 사용될 수도 있는 일부 데이터 구조에 사용되고
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있습니다.
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(*) dma_wmb();
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(*) dma_rmb();
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@ -346,24 +346,4 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
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(volatile typeof(x) *)&(x); })
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#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
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/**
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* lockless_dereference() - safely load a pointer for later dereference
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* @p: The pointer to load
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*
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* Similar to rcu_dereference(), but for situations where the pointed-to
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* object's lifetime is managed by something other than RCU. That
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* "something other" might be reference counting or simple immortality.
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*
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* The seemingly unused variable ___typecheck_p validates that @p is
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* indeed a pointer type by using a pointer to typeof(*p) as the type.
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* Taking a pointer to typeof(*p) again is needed in case p is void *.
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*/
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#define lockless_dereference(p) \
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({ \
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typeof(p) _________p1 = READ_ONCE(p); \
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typeof(*(p)) *___typecheck_p __maybe_unused; \
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smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
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(_________p1); \
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})
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#endif /* __LINUX_COMPILER_H */
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