On Fri, Aug 1, 2014 at 2:24 PM, Dennis Ferguson <dennis.c.fergu...@gmail.com> wrote: > > On 31 Jul, 2014, at 18:35 , Ryota Ozaki <ozak...@netbsd.org> wrote: > >> On Wed, Jul 30, 2014 at 11:47 AM, Ryota Ozaki <ozak...@netbsd.org> wrote: >>> >>> We may be able to use pserialize(9) instead of rwlock(9) >>> for lockless read accesses. Can we simply replace rwlock >>> with pserialize? >> >> Self-answer: no >> >> pserialize(9) requires that we don't block (sleep) in >> a critical section while rwlock(9) allows to do so. >> And there is another undocumented constraint of pserialize, >> IIUC. It requires lockless operations on data shared >> between readers and a writer. (Correct me if I'm wrong.) >> >> In the case of ifnet_list, both requirements are not >> satisfied as it is. Adapting ifnet_list is not so easy >> task, and so I think using rwlock(9) is reasonable >> at this point. > > ?? I'm confused. Which data shared between readers and writers > are you trying to protect, and were is it stored? ifnet_list is > the head of a linked list. About the only thing a reader can do > with a linked list is walk it, and about the only thing a writer > can do to a list is add something to it or delete something from > it. I can't think of anything that would block any of those > operations. How would a sleep happen? > > Or is it the case that what you are trying to do is keep anyone > from modifying anything in a struct ifnet while someone else > is reading it? If this is the problem you are trying to address > then I guess you would have to protect all struct ifnet readers, > not just those who found the ifnet structure from via the ifnet_list. > Readers mostly find ifnet structures via the pointers to them > stored in packet mbufs and in routes. > > I thought I understood what it is you wanted to accomplish, but > I think I was mistaken.
I'm sorry for confusing you. That's just an explanation why rwlock(9) cannot be simply replaced with pserialize(9). I don't think using rwlock protects all ifnet data; it can protect only ifnet_list itself. ozaki-r > > Dennis Ferguson > > > >
