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From: Nico Williams <[email protected]>
To: Heikki Linnakangas <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: [email protected]
Subject: Re: AIO writes vs hint bits vs checksums
Date: Tue, 1 Apr 2025 10:29:31 -0500
Message-ID: <Z+wGW60YBQE3n+rJ@ubby> (raw)
In-Reply-To: <[email protected]>
References: <stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m>
	<[email protected]>

On Wed, Oct 30, 2024 at 02:16:51PM +0200, Heikki Linnakangas wrote:
> Acquiring the exclusive lock in step 4 is just a way to wait for all the
> existing share-lockers to release the lock. You wouldn't need to block new
> share-lockers. We have LW_WAIT_UNTIL_FREE, which is almost what we need, but
> it currently ignores share-lockers. So doing this "properly" would require
> more changes to LWLocks. Briefly acquiring an exclusive lock seems
> acceptable though.

The problem is starvation.  For this you really want something more like
rwlocks that do not have the writer starvation problem.  But rwlocks
have other problems too, like in this case forcing readers to wait.

What you want here is something more like a barrier where readers that
did not see that the page has BM_IO_IN_PROGRESS set get to act as though
it's not set while readers that did see that the page has
BM_IO_IN_PROGRESS set don't, and the process that did set that bit gets
to wait for the first set of readers all without blocking the second set
of readers.  That's something akin to an rwlock, but better -- in fact,
it resembles RCU.

Nico
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