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From: John Baldwin <jhb@freebsd.org>
To: Scott Long <scott4long@yahoo.com>
Subject: Re: [rfc] migrate lagg to an rmlock
Date: Thu, 29 Aug 2013 12:01:03 -0400
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Cc: FreeBSD Net <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org>,
 freebsd-current@freebsd.org, Robert Watson <rwatson@freebsd.org>,
 "Alexander V. Chernikov" <melifaro@ipfw.ru>
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On Thursday, August 29, 2013 11:37:08 am Scott Long wrote:
> 
> On Aug 29, 2013, at 7:42 AM, John Baldwin <jhb@freebsd.org> wrote:
> 
> > On Saturday, August 24, 2013 10:16:33 am Robert Watson wrote:
> >> There are a number of other places in the kernel where migration to an rmlock 
> >> makes sense -- however, some care must be taken for four reasons: (1) while 
> >> read locks don't experience line contention, write locking becomes observably 
> >> e.g., rmlocks might not be suitable for tcbinfo; (2) rmlocks, unlike rwlocks, 
> >> more expensive so is not suitable for all rwlock line contention spots -- 
> >> implement reader priority propagation, so you must reason about; and (3) 
> >> historically, rmlocks have not fully implemented WITNESS so you may get less 
> >> good debugging output.  if_lagg is a nice place to use rmlocks, as 
> >> reconfigurations are very rare, and it's really all about long-term data 
> >> stability.
> > 
> > 3) should no longer be an issue.  rmlocks now have full WITNESS and assertion
> > support (including an rm_assert).
> > 
> > However, one thing to consider is that rmlocks pin readers to CPUs while the
> > read lock is held (which rwlocks do not do).
> 
> And this is not a problem for the application that we're giving it in the
> lagg driver.

That is likely true.  I was merely tweaking Robert's general guidelines re: rmlock.

-- 
John Baldwin