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From: John Baldwin <jhb@freebsd.org>
To: freebsd-current@freebsd.org
Subject: Re: [rfc] migrate lagg to an rmlock
Date: Thu, 29 Aug 2013 10:42:13 -0400
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 (bigwig.baldwin.cx); Thu, 29 Aug 2013 10:58:14 -0400 (EDT)
Cc: FreeBSD Net <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org>,
 Robert Watson <rwatson@freebsd.org>,
 "Alexander V. Chernikov" <melifaro@ipfw.ru>
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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).

-- 
John Baldwin