On Monday, August 26, 2013 2:33:41 pm Davide Italiano wrote: > On Fri, Aug 23, 2013 at 9:58 AM, John Baldwin <j...@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 <j...@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.
Well, I've thought about changing lc_lock/unlock to return a uintptr_t or void * instead of an int and then I could make rm_sleep work fine. However, that still doesn't solve the callout case. The callout case can't be fixed easily without explicitly allocating storage in the softclock thread itself. Also, I don't think you want a pointer in a lock_object. Imagine if two threads both locked and then slept on the same rm lock in succession while waiting for a wakeup. You would have two trackers to keep track of, but only one pointer in the lock_object. I'm not sure you need to revert your commit. It should pretty much panic instantly if someone tries to use it with a read lock instead of a write lock, even without INVARIANTS. -- John Baldwin _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
