Re: valgrind versus pg_atomic_init()

From: Andres Freund <andres(at)anarazel(dot)de>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: valgrind versus pg_atomic_init()
Date: 2020-06-15 04:16:20
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On 2020-06-14 18:55:27 -0700, Noah Misch wrote:
> Does something guarantee the write will be globally-visible by the time the
> first concurrent accessor shows up?

The function comments say:

* Has to be done before any concurrent usage..
* No barrier semantics.

> (If not, one could (a) do an unlocked ptr->value=0, then the atomic
> write, or (b) revert and improve the suppression.) I don't doubt it's
> fine for the ways PostgreSQL uses atomics today, which generally
> initialize an atomic before the concurrent-accessor processes even
> exist.

I think it's unlikely that there are cases where you could safely
initialize the atomic without needing some form of synchronization
before it can be used. If a barrier were needed, what'd guarantee the
concurrent access happened after the initialization in the first place?


Andres Freund

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