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From: Andres Freund <[email protected]>
To: Pavel Borisov <[email protected]>
Cc: Alexander Korotkov <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
Date: Fri, 7 Mar 2025 12:38:00 -0500
Message-ID: <tsznvs62vmho3qlswushbfy35gzkcyrv6bxk67g2pfbjpruepj@3ts2xiafyxxb> (raw)
In-Reply-To: <CALT9ZEHDC1jz3z54hJ2_PvL+6D+u4niXGVxcBvbhHaNeGOXOhw@mail.gmail.com>
References: <CAPpHfdudejddHLLw=Uk_7pZ=1g2SZXfwubSgmWUhJ7fBUtR9rg@mail.gmail.com>
	<2muwyx6a5vojkg7iegknhnkcch3lfxptsxk7icwuh7szkvvu2y@vc3ukkfvnu6i>
	<CAPpHfdvucbQ9w7_eJYefUUp6w-WZUes+SRfq+GKfhukhs7kLqg@mail.gmail.com>
	<CALT9ZEHDC1jz3z54hJ2_PvL+6D+u4niXGVxcBvbhHaNeGOXOhw@mail.gmail.com>

Hi,

On 2025-03-07 21:08:34 +0400, Pavel Borisov wrote:
> On Fri, 7 Mar 2025 at 20:40, Alexander Korotkov <[email protected]> wrote:
> > Yes, there surely should be a memory barrier on another side.  But
> > does __ATOMIC_SEQ_CST works as a barrier for the regular read/write
> > operations on the same side?
> According to a reference posted by Andres [1]:
> "Within a thread of execution, accesses (reads and writes) through
> volatile lvalues cannot be reordered past observable side-effects
> (including other volatile accesses) that are separated by a sequence
> point within the same thread, but this order is not guaranteed to be
> observed by another thread, since volatile access does not establish
> inter-thread synchronization."

How is volatile relevant here?


> Also: "as soon as atomic operations that are not tagged
> memory_order_seq_cst enter the picture, the sequential consistency is
> lost"

Sequential consistency is lost, but that does *NOT* mean that acquire/release
guarantees that are also guaranteed by ATOMIC_SEQ_CST are lost.

Greetings,

Andres Freund

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