From: Claudio Jeker <cjeker@diehard.n-r-g.com>
Subject: Re: [PATCH] convert mpl ticket lock to anderson's lock
To: Theo de Raadt <deraadt@openbsd.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>, tech@openbsd.org
Date: Tue, 17 Feb 2026 21:06:52 +0100

On Tue, Feb 17, 2026 at 12:25:16PM -0700, Theo de Raadt wrote:
> Theo de Raadt <deraadt@openbsd.org> wrote:
> 
> > > Per that post, the primary problem concerns page allocation and the
> > > way mutexes are implemented
> > 
> > While looking at things in the pagedaemon, I've become concerned
> > about how uvm_lock_pageq() is used
> > 
> > Look at how uvm_pageactivate() and uvm_pagedeactivate() must be called
> > unlocked
> > 
> > So locked-callers unlock temporarily to call them, resulting in
> > very strange unlock/lock/tiny-operation/unlock/lock sequences.
> > 
> > Without the locks the tiny-operation would be instantenous but this
> > design means there is a tremendous bubble if someone else wanted the lock.
> > 
> > I think these low-level changes in locking and mutexes will have little
> > effect if we are using them extremely poorly, which we are.
> 
> This is why.
> 
> revision 1.184
> date: 2025/12/10 08:38:18;  author: mpi;  state: Exp;  lines: +44 -37;  commitid: ACX7PGcPcpfRHo20;
> Push `pageqlock' dances inside uvm_page{de,}activate() & uvm_pagewire().
> 
> Tested during multiple bulks on amd64, i386, arm64 and sparc64 by jca@,
> phessler@ and sthen@.
>  
> 
> 
> This pushing of the lock into the lower level functions, in turn requires that
> many higher level callers have to unlock, in order to a function which now locks,
> and upon return, relock for their own needs.
> 
> I don't think think that reduces contention but only hides it.  But it
> definately increases latency for the specific caller because someone else
> wins the mutex during the two release windows.  Parallelism may have increased,
> but latency of critical functions calling these will suffer.  Some of these
> critical functions if they did't do this dance, would be finished quicker and
> the system would be less contended.
> 
> To me this looks like deck chairs succesfully rearranged.
> 

I actually think this diff introduced some locking issues where objects
get modified in multiple steps instead of atomically. If two cpus run in
lock step the result could be a corrupt object.

See my other mail regarding the wire count assert.
-- 
:wq Claudio