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Commitbd5132d

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Introduce atomic read/write functions with full barrier semantics.
Writing correct code using atomic variables is often difficult dueto the memory barrier semantics (or lack thereof) of the underlyingoperations. This commit introduces atomic read/write functionswith full barrier semantics to ease this cognitive load. Forexample, some spinlocks protect a single value, and these newfunctions make it easy to convert the value to an atomic variable(thus eliminating the need for the spinlock) without modifying thebarrier semantics previously provided by the spinlock. Since thesefunctions may be less performant than the other atomic reads andwrites, they are not suitable for every use-case. However, using asingle atomic operation with full barrier semantics may be moreperformant in cases where a separate explicit barrier wouldotherwise be required.The base implementations for these new functions are atomicexchanges (for writes) and atomic fetch/adds with 0 (for reads).These implementations can be overwritten with better architecture-specific versions as they are discovered.This commit leaves converting existing code to use these newfunctions as a future exercise.Reviewed-by: Andres Freund, Yong Li, Jeff DavisDiscussion:https://postgr.es/m/20231110205128.GB1315705%40nathanxps13
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2 files changed

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‎src/include/port/atomics.h

Lines changed: 58 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -237,6 +237,26 @@ pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
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returnpg_atomic_read_u32_impl(ptr);
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}
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/*
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* pg_atomic_read_membarrier_u32 - read with barrier semantics.
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*
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* This read is guaranteed to return the current value, provided that the value
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* is only ever updated via operations with barrier semantics, such as
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* pg_atomic_compare_exchange_u32() and pg_atomic_write_membarrier_u32().
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* While this may be less performant than pg_atomic_read_u32(), it may be
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* easier to reason about correctness with this function in less performance-
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* sensitive code.
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*
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* Full barrier semantics.
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*/
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staticinlineuint32
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pg_atomic_read_membarrier_u32(volatilepg_atomic_uint32*ptr)
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{
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AssertPointerAlignment(ptr,4);
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returnpg_atomic_read_membarrier_u32_impl(ptr);
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}
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/*
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* pg_atomic_write_u32 - write to atomic variable.
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*
@@ -274,6 +294,26 @@ pg_atomic_unlocked_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
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pg_atomic_unlocked_write_u32_impl(ptr,val);
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}
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/*
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* pg_atomic_write_membarrier_u32 - write with barrier semantics.
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*
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* The write is guaranteed to succeed as a whole, i.e., it's not possible to
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* observe a partial write for any reader. Note that this correctly interacts
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* with both pg_atomic_compare_exchange_u32() and
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* pg_atomic_read_membarrier_u32(). While this may be less performant than
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* pg_atomic_write_u32(), it may be easier to reason about correctness with
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* this function in less performance-sensitive code.
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*
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* Full barrier semantics.
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*/
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staticinlinevoid
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pg_atomic_write_membarrier_u32(volatilepg_atomic_uint32*ptr,uint32val)
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{
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AssertPointerAlignment(ptr,4);
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pg_atomic_write_membarrier_u32_impl(ptr,val);
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}
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/*
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* pg_atomic_exchange_u32 - exchange newval with current value
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*
@@ -427,6 +467,15 @@ pg_atomic_read_u64(volatile pg_atomic_uint64 *ptr)
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returnpg_atomic_read_u64_impl(ptr);
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}
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staticinlineuint64
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pg_atomic_read_membarrier_u64(volatilepg_atomic_uint64*ptr)
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{
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#ifndefPG_HAVE_ATOMIC_U64_SIMULATION
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AssertPointerAlignment(ptr,8);
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#endif
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returnpg_atomic_read_membarrier_u64_impl(ptr);
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}
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staticinlinevoid
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pg_atomic_write_u64(volatilepg_atomic_uint64*ptr,uint64val)
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{
@@ -436,6 +485,15 @@ pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
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pg_atomic_write_u64_impl(ptr,val);
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}
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staticinlinevoid
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pg_atomic_write_membarrier_u64(volatilepg_atomic_uint64*ptr,uint64val)
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{
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#ifndefPG_HAVE_ATOMIC_U64_SIMULATION
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AssertPointerAlignment(ptr,8);
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#endif
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pg_atomic_write_membarrier_u64_impl(ptr,val);
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}
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staticinlineuint64
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pg_atomic_exchange_u64(volatilepg_atomic_uint64*ptr,uint64newval)
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{

‎src/include/port/atomics/generic.h

Lines changed: 36 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -243,6 +243,24 @@ pg_atomic_sub_fetch_u32_impl(volatile pg_atomic_uint32 *ptr, int32 sub_)
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}
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#endif
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#if !defined(PG_HAVE_ATOMIC_READ_MEMBARRIER_U32)&& defined(PG_HAVE_ATOMIC_FETCH_ADD_U32)
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#definePG_HAVE_ATOMIC_READ_MEMBARRIER_U32
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staticinlineuint32
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pg_atomic_read_membarrier_u32_impl(volatilepg_atomic_uint32*ptr)
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{
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returnpg_atomic_fetch_add_u32_impl(ptr,0);
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}
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#endif
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#if !defined(PG_HAVE_ATOMIC_WRITE_MEMBARRIER_U32)&& defined(PG_HAVE_ATOMIC_EXCHANGE_U32)
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#definePG_HAVE_ATOMIC_WRITE_MEMBARRIER_U32
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staticinlinevoid
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pg_atomic_write_membarrier_u32_impl(volatilepg_atomic_uint32*ptr,uint32val)
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{
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(void)pg_atomic_exchange_u32_impl(ptr,val);
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}
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#endif
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#if !defined(PG_HAVE_ATOMIC_EXCHANGE_U64)&& defined(PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U64)
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#definePG_HAVE_ATOMIC_EXCHANGE_U64
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staticinlineuint64
@@ -399,3 +417,21 @@ pg_atomic_sub_fetch_u64_impl(volatile pg_atomic_uint64 *ptr, int64 sub_)
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returnpg_atomic_fetch_sub_u64_impl(ptr,sub_)-sub_;
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}
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#endif
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#if !defined(PG_HAVE_ATOMIC_READ_MEMBARRIER_U64)&& defined(PG_HAVE_ATOMIC_FETCH_ADD_U64)
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#definePG_HAVE_ATOMIC_READ_MEMBARRIER_U64
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staticinlineuint64
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pg_atomic_read_membarrier_u64_impl(volatilepg_atomic_uint64*ptr)
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{
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returnpg_atomic_fetch_add_u64_impl(ptr,0);
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}
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#endif
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#if !defined(PG_HAVE_ATOMIC_WRITE_MEMBARRIER_U64)&& defined(PG_HAVE_ATOMIC_EXCHANGE_U64)
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#definePG_HAVE_ATOMIC_WRITE_MEMBARRIER_U64
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staticinlinevoid
433+
pg_atomic_write_membarrier_u64_impl(volatilepg_atomic_uint64*ptr,uint64val)
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{
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(void)pg_atomic_exchange_u64_impl(ptr,val);
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}
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#endif

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