From: "Ted Unangst" <tedu@tedunangst.com>
Subject: Re: producer/consumer locking
To: "David Gwynne" <david@gwynne.id.au>
Cc: tech@openbsd.org
Date: Sun, 04 May 2025 12:56:44 -0400

On 2025-05-04, David Gwynne wrote:

> CAVEATS
>      Updates must be produced infrequently enough to allow time for consumers
>      to be able to get a consistent read without looping too often.

This was my first thought. I'm worried we'll spin and tank performance, but
won't have any visibility. Maybe there should be a count of failures?

> pc_sprod_leave() marks the end of a single producer critical section for
     the pcl producer/consumer lock.  The gen argument must be the value
     returned from the preceding pc_sprod_enter() call.

I don't think there's any need for the leave functions to take gen.
Just increment the stored gen. Multiple consumers can't change it while
it's odd.

+	if (gen & 1) {
+		do {
+			CPU_BUSY_CYCLE();
+			gen = pcl->pcl_gen;
+		} while (gen & 1);
+	} else if (gen == *genp)
+		return (0);

This will be cleaner if you put the else first. Then make it a while loop.