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From: Alvaro Herrera <[email protected]>
To: Tom Lane <[email protected]>
Cc: Alexander Lakhin <[email protected]>
Cc: Bharath Rupireddy <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Jeff Davis <[email protected]>
Cc: [email protected]
Cc: Jaime Casanova <[email protected]>
Subject: Re: LogwrtResult contended spinlock
Date: Mon, 1 Jul 2024 16:35:02 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 2024-Jul-01, Tom Lane wrote:

> Alvaro Herrera <[email protected]> writes:
> >> because the failed assertion is:
> >> #ifndef PG_HAVE_ATOMIC_U64_SIMULATION
> >>     AssertPointerAlignment(&currval, 8);
> >> #endif
> 
> Perhaps this assertion is what is wrong?  If the platform has no
> native 8-byte alignment requirement, why do we think that atomics
> need it?

Oh, that's a good question.  TBH I just copied the assertion from the
other routines for 64-bit variables in the same file.  But I think
that's correct.  We're gating the assertion on _not_ having emulation,
which must mean we have native atomics; on MSVC, if I read the #ifdef
maze correctly, that's implemented using _InterlockedCompareExchange,
whose docs state:

: The variables for this function must be aligned on a 64-bit boundary;
: otherwise, this function will behave unpredictably on multiprocessor x86
: systems and any non-x86 systems. See _aligned_malloc.
https://learn.microsoft.com/en-us/windows/win32/api/winnt/nf-winnt-interlockedcompareexchange64

So I think the assertion is correct.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"[PostgreSQL] is a great group; in my opinion it is THE best open source
development communities in existence anywhere."                (Lamar Owen)

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