On Fri, Jul 11, 2014 at 1:56 PM, Frank Filz <ffilzlnx@xxxxxxxxxxxxxx> wrote: >> On Fri, 11 Jul 2014 10:31:26 -0700 >> "Frank Filz" <ffilzlnx@xxxxxxxxxxxxxx> wrote: >> >> > > The current enforcement of deny modes is both inefficient and >> > > scattered across several places, which makes it hard to guarantee >> > > atomicity. The inefficiency is a problem now, and the lack of >> > > atomicity will mean races >> > once >> > > the client_mutex is removed. >> > > >> > > First, we address the inefficiency. We have to track deny modes on a >> > > per- stateid basis to ensure that open downgrades are sane, but when >> > > the server goes to enforce them it has to walk the entire list of >> > > stateids and check against each one. >> > > >> > > Instead of doing that, maintain a per-nfs4_file deny mode. When a >> > > file is opened, we simply set any deny bits in that mode that were >> > > specified in >> > the >> > > OPEN call. We can then use that unified deny mode to do a simple >> > > check to see whether there are any conflicts without needing to walk >> > > the entire stateid list. >> > > >> > > The only time we'll need to walk the entire list of stateids is when >> > > a >> > stateid >> > > that has a deny mode on it is being released, or one is having its >> > > deny >> > mode >> > > downgraded. In that case, we must walk the entire list and >> > > recalculate the fi_share_deny field. Since deny modes are pretty >> > > rare today, this should >> > be >> > > very rare under normal workloads. >> > >> > What we do in Ganesha to avoid walking the list of stateids on release >> > is maintain the effective deny (and access) mode not at bits, but as a >> > counter for each bit. Thus, to remove a SHARE_ACCESS_READ | >> > SHARE_DENY_WRITE, you decrement the counts for access_read and >> deny_write. >> > >> > Frank >> > >> > >> >> Sure, that's another possibility that I considered, but I didn't want to > be >> bothered with having to add counters for deny modes. In practice there are >> *no* clients that use them (aside from pynfs and maybe the semi-mythical >> Windows v4.1 client). You don't need counters for deny modes. There can only be 1 of each, since any deny mode has to be part of an OPEN that has at least one non-zero share access mode. So a single bit for each should be fine. >> >> With this scheme, deny mode enforcement is pretty darned efficient, >> particularly in the common case where there are no deny modes to enforce. >> >> Any penalty for the use of deny modes is generally paid during the CLOSE > or >> OPEN_DOWNGRADE on behalf of the client that's using them. >> Any RPC from a client that's not won't need to do any extra work (aside > from >> maybe spinning on the fi_lock while another client is having to > recalculate the >> fi_share_deny). > > Good point. > > Whatever happened to Pavel Shilovsky's O_DENY patch set? I was looking > forward to that for allowing Ganesha and Samba share reservations to more > fully interact with each other... > > Frank > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-nfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
