From: Alexandr Nedvedicky <sashan@fastmail.net>
Subject: Re: net lock kernel lock order
To: Alexander Bluhm <alexander.bluhm@gmx.net>
Cc: tech@openbsd.org
Date: Fri, 5 Jan 2024 14:03:14 +0100

Hello,

On Fri, Jan 05, 2024 at 01:26:15PM +0100, Alexander Bluhm wrote:
> Hi,
> 
> We have code that takes kernel lock just before net lock.  As net
> lock is a rwlock that releases kernel lock while sleeping, there
> is never deadlock due to lock ordering.
> 
> The order kernel lock -> net lock is suboptimal.  If net lock is
> held by another thread the following happens:
> 
> take kernel lock -> try net lock -> release kernel lock -> sleep
> -> take net lock -> take kernel lock
> 
> If we change the order we get a less expensive sequence:
> 
> try net lock -> sleep -> take net lock -> take kernel lock
> 
> ok?

    yes, the change makes sense to me.

OK sashan