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Commit213c7df

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Impose a full barrier in generic-xlc.h atomics functions.
pg_atomic_compare_exchange_*_impl() were providing only the semantics ofan acquire barrier. Buildfarm members hornet and mandrill revealed thisdeficit beginning with commit008608b.While we have no report of symptoms in 9.5, we can't rule out thepossibility of certain compilers, hardware, or extension code relying onthese functions' specified barrier semantics. Back-patch to 9.5, wherecommitb64d92f introduced atomics.Reviewed by Andres Freund.
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‎src/include/port/atomics/generic-xlc.h

Lines changed: 22 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -40,12 +40,22 @@ static inline bool
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pg_atomic_compare_exchange_u32_impl(volatilepg_atomic_uint32*ptr,
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uint32*expected,uint32newval)
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{
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boolret;
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/*
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* atomics.h specifies sequential consistency ("full barrier semantics")
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* for this interface. Since "lwsync" provides acquire/release
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* consistency only, do not use it here. GCC atomics observe the same
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* restriction; see its rs6000_pre_atomic_barrier().
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*/
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__asm__ __volatile__ ("sync \n" :::"memory");
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/*
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* XXX: __compare_and_swap is defined to take signed parameters, but that
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* shouldn't matter since we don't perform any arithmetic operations.
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*/
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boolret=__compare_and_swap((volatileint*)&ptr->value,
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(int*)expected, (int)newval);
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ret=__compare_and_swap((volatileint*)&ptr->value,
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(int*)expected, (int)newval);
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/*
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* xlc's documentation tells us:
@@ -63,6 +73,10 @@ pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
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staticinlineuint32
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pg_atomic_fetch_add_u32_impl(volatilepg_atomic_uint32*ptr,int32add_)
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{
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/*
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* __fetch_and_add() emits a leading "sync" and trailing "isync", thereby
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* providing sequential consistency. This is undocumented.
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*/
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return__fetch_and_add((volatileint*)&ptr->value,add_);
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}
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@@ -73,8 +87,12 @@ static inline bool
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pg_atomic_compare_exchange_u64_impl(volatilepg_atomic_uint64*ptr,
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uint64*expected,uint64newval)
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{
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boolret=__compare_and_swaplp((volatilelong*)&ptr->value,
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(long*)expected, (long)newval);
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boolret;
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__asm__ __volatile__ ("sync \n" :::"memory");
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ret=__compare_and_swaplp((volatilelong*)&ptr->value,
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(long*)expected, (long)newval);
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__isync();
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