> 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. You can have any number of: OPEN SHARE_ACCESS_READ, SHARE_DENY_WRITE... It's the share reservation equivalent of a read lock... Frank > >> > >> 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
