linux-kernel - Re: Oi. NFS people. Read this.

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Date:	Wed, 07 May 2008 15:10:27 -0700
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: Oi.  NFS people.  Read this.

On Wed, 2008-05-07 at 14:00 -0600, Matthew Wilcox wrote:
> On Wed, May 07, 2008 at 12:44:48PM -0700, Linus Torvalds wrote:
> > On Wed, 7 May 2008, Matthew Wilcox wrote:
> > > 
> > > One patch I'd still like Yanmin to test is my one from yesterday which
> > > removes the BKL from fs/locks.c.
> > 
> > And I'd personally rather have the network-fs people test and comment on 
> > that one ;)
> > 
> > I think that patch is worth looking at regardless, but the problems with 
> > that one aren't about performance, but about what the implications are for 
> > the filesystems (if any)...
> 
> Oh, well, they don't seem interested.

Poor timing: we're all preparing for and travelling to the annual
Connectathon interoperability testing conference which starts tomorrow.

> I can comment on some of the problems though.
> 
> fs/lockd/svcsubs.c, fs/nfs/delegation.c, fs/nfs/nfs4state.c,
> fs/nfsd/nfs4state.c all walk the i_flock list under the BKL.  That won't
> protect them against locks.c any more.  That's probably OK for fs/nfs/*
> since they'll be protected by their own data structures (Someone please
> check me on that?), but it's a bad idea for lockd/nfsd which are walking
> the lists for filesystems.

Yes. fs/nfs is just reusing the code in fs/locks.c in order to track the
locks it holds on the server. We could alternatively have coded a
private lock implementation, but this seemed easier.

> Are we going to have to export the file_lock_lock?  I'd rather not.  But
> we need to keep nfsd/lockd from tripping over locks.c.
> 
> Maybe we could come up with a decent API that lockd could use?  It all
> seems a bit complex at the moment ... maybe lockd should be keeping
> track of the locks it owns anyway (since surely the posix deadlock
> detection code can't work properly if it's just passing all the locks
> through).

I'm not sure what you mean when you talk about lockd keeping track of
the locks it owns. It has to keep those locks on inode->i_flock in order
to make them visible to the host filesystem...

All lockd really needs, is the ability to find a lock it owns, and then
obtain a copy. As for the nfs client, I suspect we can make do with
something similar...

Cheers
  Trond

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