On Tue, Apr 01, 2008, Jeff Roberson wrote: > On Wed, 2 Apr 2008, Max Laier wrote: > > >On Tuesday 01 April 2008 22:31:55 Attilio Rao wrote: > >>attilio 2008-04-01 20:31:55 UTC > >> > >> FreeBSD src repository > >> > >> Modified files: > >> sys/kern kern_rwlock.c > >> sys/sys rwlock.h > >> Log: > >> Add rw_try_rlock() and rw_try_wlock() to rwlocks. > >> These functions try the specified operation (rlocking and wlocking) > >>and true is returned if the operation completes, false otherwise. > > > >hmmm ... I'm certainly missing something here, but what's a possible > >usecase for these? It seems there is not much you can do if you can't > >obtain a rw_lock. I can understand the need for sx_try_* where you want > >to avoid sleeping, but I can't figure out the need for it on a locking > >primitive that will only spin or wait (not 100% sure about the > >terminology here). This is especially strange for rw_try_wlock, unless > >you plan to sleep manually on fail. But then again you'd have a good > >chance that you have to do it over and over again if timing is > >unfortunate. > > I asked for it. We have a try operation for mtx already. I was > experimenting with converting some code to use rwlocks from mtx and it > required it. The specific case is in the softdep code where it uses > trylock to avoid deadlocking. With trylock you can violate the lockorder.
One reason that many systems don't provide a rw_try_wlock() primitive is that a constant stream of readers can easily starve threads attempting to acquire the write lock without waiting. There are probably situations where this isn't a problem, though, e.g., if readers rarely hold the lock for a long time... _______________________________________________ cvs-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/cvs-all To unsubscribe, send any mail to "[EMAIL PROTECTED]"
