Re: Atomics in localbuf.c

Hi

On March 5, 2020 12:42:06 PM PST, Antonin Houska <ah@cybertec.at> wrote:
>Andres Freund <andres@anarazel.de> wrote:
>
>> On March 5, 2020 9:21:55 AM PST, Antonin Houska <ah@cybertec.at>
>wrote:
>> >What's the reason to use pg_atomic...read_...() and
>> >pg_atomic...write_...()
>> >functions in localbuf.c?
>> >
>> >It looks like there was an intention not to use them
>> >
>>
>>https://www.postgresql.org/message-id/CAPpHfdtfr3Aj7xJonXaKR8iY2p8uXOQ%2Be4BMpMDAM_5R4OcaDA%40mail.gmail.com
>> >
>> >but the following discussion does not explain the decision to use
>them.
>>
>> Read/write don't trigger locked/atomic operations. They just
>guarantee that
>> you're not gonna read/write a torn value. Or a cached one. Since
>> local/shared buffers share the buffer header definition, we still
>have to
>> use proper functions to access the atomic variables.
>
>Sure, the atomic operations are necessary for shared buffers, but I
>still
>don't understand why they are needed for *local* buffers - local
>buffers their
>headers (BufferDesc) in process local memory, so there should be no
>concerns
>about concurrent access.
>
>Another thing that makes me confused is this comment in
>InitLocalBuffers():
>
>    /*
>     * Intentionally do not initialize the buffer's atomic variable
>     * (besides zeroing the underlying memory above). That way we get
>     * errors on platforms without atomics, if somebody (re-)introduces
>     * atomic operations for local buffers.
>     */
>
>That sounds like there was an intention not to use the atomic functions
>in
>localbuf.c, but eventually they ended up there. Do I still miss
>something?

Again, the read/write functions do not imply atomic instructions.

Ants
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