Re: [rfc] migrate lagg to an rmlock

On Aug 29, 2013, at 7:42 AM, John Baldwin <jhb_at_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.

Scott