From: Alexander Bluhm <bluhm@openbsd.org>
Subject: Re: unlock tcp drop sysctl
To: Vitaliy Makkoveev <mvs@openbsd.org>
Cc: tech@openbsd.org
Date: Thu, 9 Jan 2025 19:59:25 +0100

On Thu, Jan 09, 2025 at 07:00:42PM +0300, Vitaliy Makkoveev wrote:
> Would you like to split tcp_sysctl() with tcp_sysctl_locked() and
> start slightly moving some paths out of sysctl lock? 

If I understand correctly, PR_MPSYSCTL prevents kernel lock, does
not serialize sysctl calls, and does not wire the memory for copyin().

TCP sysctl does not care about kernel lock.  It uses either atomic
integers, mutex protection or exclusive net lock.  Calling copyin()
or copyout() without wired memory while holding the net lock is bad
due to NFS.

I think is easier to remove net lock sysctl per sysctl and then
switch the whole tcp_sysctl() instead of introducing tcp_sysctl_locked().

bluhm