Date: Mon, 13 Apr 2009 11:46:21 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-fs@freebsd.org Subject: Re: integrating nfsv4 locking with nlm and local locking Message-ID: <200904131146.21640.jhb@freebsd.org> In-Reply-To: <Pine.GSO.4.63.0904091433590.24215@muncher.cs.uoguelph.ca> References: <Pine.GSO.4.63.0904091433590.24215@muncher.cs.uoguelph.ca>
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On Thursday 09 April 2009 3:04:37 pm Rick Macklem wrote: > My nfsv4 server currently does VOP_ADVLOCK() with the non-blocking F_SETLK > type and I had thought that was sufficient, but I now realize (thanks to > a recent post by Zachary Loafman) that this breaks when a delegation for > the file is issued to a client. (When a delegation for a file is issued > to a client, it can do byte range locking locally, and the server doesn't > know about these to do VOP_ADVLOCK() on the server machine.) > > I believe that Zachary would like to discuss a more general solution, > including how to handle Open/Share locks, but in the meantime I'd like to > solve this specific case in as simple a way as possible. > > Basically, I need a way to make sure delegations for a file don't exist > when local byte range locking or locking via the NLM is being done on > the file. > > The simplest thing I can think of is the following: > When VOP_ADVLOCK() is called for a file (outside of the nfsv4 server), > do two things: > 1 - Make sure any outstanding delegations are recalled. > I already have a function that does this, so it is a matter > of figuring out where to put the call(s). > 2 - Set a flag on the vnode, so that my nfsv4 server knows not to > issue another delegation for that file. > (I could test for locks via VOP_ADVLOCK() before issuing a > delegation, but that has two problems.) > 1 - Since the vnode is unlocked for VOP_ADVLOCK(), there could > be a race where the nfsv4 server issues a delegation > between the time outstanding delegations are recalled at > #1 above and the VOP_ADVLOCK() sets the lock that I would > see during the test. > 2 - It would have to keep checking for a lock and might issue > a delegation at a point where no lock is held, but one > will be acquired soon, forcing the delegation recall. > (It's much easier to not issue a delegation than recall > one.) > Once this flag is set, I think it would be ok if the flag > remains set until the vnode is recycled, since it seems > fairly likely that, once byte range locking is done on a > file, more will happen. > (If people were agreeable to the vnode flag, it looks like > a VV_xxx flag would make more sense than a VI_xxx one. I > think an atomic_set_int() would be sufficient to set it, > even though the vnode lock isn't held?) You have to hold the vnode lock to set a VV flag always. Even if you do an atomic operation to set your flag, another thread might be setting a flag at the same time using non-atomic ops and could clobber your change (if it does a read-modify-write and reads a value that pre-dates your atomic_set_int() but its write posts after your write). -- John Baldwin
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