Date: Mon, 26 Aug 2013 11:33:41 -0700 From: Davide Italiano <davide@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r254703 - in head: share/man/man9 sys/sys Message-ID: <CACYV=-ELQNPR3_Ok12J5UbUpROcr5oEVv6ndnnpnU781tyKgrw@mail.gmail.com> In-Reply-To: <201308231258.50969.jhb@freebsd.org> References: <201308231412.r7NECdG7081565@svn.freebsd.org> <201308231051.08997.jhb@freebsd.org> <CACYV=-H_CaCf9Ob=XW1fPZtPwNSYfdVdn2eNxnEV-ta7HiNhUw@mail.gmail.com> <201308231258.50969.jhb@freebsd.org>
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On Fri, Aug 23, 2013 at 9:58 AM, John Baldwin <jhb@freebsd.org> wrote: > 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 I see. I would really like to go for a clean solution if possible, and if the timeframe for 10 doesn't allow this just revert the commit until a better solution would be available. FWIW, I pondered a bit about this and the only way I was able to think is that of augmenting 'struct lock_object' with a 'void *arg' field that in this case could be used to store a pointer to something, which in this case is a pointer to a rmtracker object, and this could allow easily to retrieve the needed information (as far as I see something similar is done to store WITNESS information). This, OTOH, could be overkill just to fix this case though. Thanks, -- Davide "There are no solved problems; there are only problems that are more or less solved" -- Henri Poincare
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