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From: John Baldwin <jhb@freebsd.org>
To: Davide Italiano <davide@freebsd.org>
Subject: Re: svn commit: r254703 - in head: share/man/man9 sys/sys
Date: Fri, 23 Aug 2013 12:58:50 -0400
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On Friday, August 23, 2013 11:29:45 am Davide Italiano wrote:
> On Fri, Aug 23, 2013 at 4:51 PM, John Baldwin <jhb@freebsd.org> wrote:
> > On Friday, August 23, 2013 10:12:39 am Davide Italiano wrote:
> >> Author: davide
> >> Date: Fri Aug 23 14:12:39 2013
> >> New Revision: 254703
> >> URL: http://svnweb.freebsd.org/changeset/base/254703
> >>
> >> Log:
> >>   Introduce callout_init_rm() so that callouts can be used in conjunction
> >>   with rmlocks. This works only with non-sleepable rm because handlers run
> >>   in SWI context. While here, document the new KPI in the timeout(9)
> >>   manpage.
> >
> > It also only works with exclusive locks.  (lc_unlock/lc_lock only handle
> > write locks for rmlocks).
> >
> > --
> > John Baldwin
> 
> Thanks for pointing out this.
> I think it would be nice to have lc_lock/lc_unlock working both for
> shared and exclusive locks but I'm not 100% sure about all the
> implications/complications. From what I see for rwlocks asserting if a
> lock is held in read-mode is really cheap (check against a flag) while
> for rmlocks the assertion relies on traversing the tracker list for
> the rmlock so I'm worried this operation could be expensive. What's
> your opinion about?

The much bigger problem is you need an rmtracker object to pass to the
lock/unlock routines.

You could make this work hackishly in the callout case by special casing
rm locks that use read locking and using a tracker on softclock's stack,
but it is much harder to fix this for the rm_sleep() case where the
sequence is lc_unlock/lc_lock.

-- 
John Baldwin